Odd Low-Pitched Buzz Noise from Engine

08XC90V8 · Dec 14, 2020

Car in context: 2008 XC90 V8 AWD w/68k miles, recent Haldex AOC job done which required rear exhaust assembly removal (see note about broken stud below)

This issue started last night, when I did a cold start (cold is relative, the XC90 stays in the garage the entire time) and heard this odd, low-pitched, "heavy" buzzing noise that came as soon as the engine turned on and went away within 2-3 seconds. It's not a rattle, it's a low-pitched, "heavy" buzz. It can be felt and heard well in the cabin. For context, I did the Haldex AOC job on the car a few weeks ago, and I had to drop the exhaust and the driveshaft for that. In the process, one of the studs on the flange that connect the downpipe to the rest of the rear exhaust assembly broke off and I had to heat it up with a blowtorch and pound it out with a hammer. It came out fine, and I used a new washer, bolt, and nut to replace the stud. I didn't mess with the other stud, which was perfectly fine. I ensured that this flange was tight and sealed, and tightened it down real good.

I'm not sure if this is an exhaust issue, but I'm leaning towards it being one. It also seems to be coming from that area I'm talking about where that flange I messed with resides. This issue does NOT happen after the cold start, looks like it goes away as the engine and everything warms up. It ONLY happens on cold starts, it happened again earlier today. No issues as far as I can tell with exhaust while driving, so it looks like it's consolidated to startup. And I know it has only started since last night. Nothing else has been done to the car, but it had been sitting in the garage for about 7-9 days before I started it up, heard the noise, and carried on driving.

I'll try to get a video of it later tomorrow. Happy Holidays, everyone!

Masscomguru · Dec 14, 2020

When I had my 3.2 and it was time to replace the front muffler, the muffler was cracked at the very front and top. I heard a similar sound on that until the rear of the muffler blew out and I needed to replace it. I was not able to track down the noise I was hearing but it was gone with the replacement of the muffler. I did not know it was cracked until the time it was pulled. I think you are probably on the right track.

08XC90V8 · Dec 14, 2020

Masscomguru said:
When I had my 3.2 and it was time to replace the front muffler, the muffler was cracked at the very front and top. I heard a similar sound on that until the rear of the muffler blew out and I needed to replace it. I was not able to track down the noise I was hearing but it was gone with the replacement of the muffler. I did not know it was cracked until the time it was pulled. I think you are probably on the right track.

Ah. What part of the exhaust is the muffler? I know the V8 has a different factory exhaust than the 3.2, and there's a sort of oval-like "box" under the driveshaft closer to the downpipe and then a massive metal box with the tailpipes at the end. Asking so I can attempt a visual inspection.

Masscomguru · Dec 14, 2020

Actually from the cat's back, the exhaust is the same. You will have to replace the front muffler at some point. They all go. The front muffler is a straight through glasspack. it cracks on the top and the rear typically rusts and blows out. If you search you will see plenty of examples of 3.2 and V8 failed mufflers. The rear is always fine.

08XC90V8 · Dec 14, 2020

Masscomguru said:
Actually from the cat's back, the exhaust is the same. You will have to replace the front muffler at some point. They all go. The front muffler is a straight through glasspack. it cracks on the top and the rear typically rusts and blows out. If you search you will see plenty of examples of 3.2 and V8 failed mufflers. The rear is always fine.

Would that theory still hold given that the sound goes away when the engine (and exhaust) is warmed up? And what causes mufflers to break? There's absolutely ZERO rust on the car, including the undercarriage. It's been garaged its whole life, only 68k miles on it so far.

Masscomguru · Dec 14, 2020

As the exhaust heats up, metal expands, reducing the size of the cracks, openings. In-terms of rusting out/breaking apart, seems to be from the inside out. You can garage a car all you want, you still need to drive it outside the garage. They just all do it at some point. It's just a theory, but worth investigating.

08XC90V8 · Dec 14, 2020

Masscomguru said:
As the exhaust heats up, metal expands, reducing the size of the cracks, openings. In-terms of rusting out/breaking apart, seems to be from the inside out. You can garage a car all you want, you still need to drive it outside the garage. They just all do it at some point. It's just a theory, but worth investigating.

Interesting. I'm not sure how I'll be able to diagnose the exhaust (inspect for cracks from under the car to the best of my ability, I guess?) but I have been planning on doing a rear muffler delete for a long time now.

Hopefully the issue is with the rearward muffler so I can convince myself cutting it out and replacing with a Borla XR-1 was a "repair", and not a sound upgrade like it really is

Thanks for the help.

08XC90V8 · Dec 15, 2020

Update: sound, of course as soon as I try to capture it on video, has gone away. No idea what was causing it.

08XC90V8 · Dec 22, 2020

So I was able to catch the sound happening again earlier today. The car had been off and sitting in the garage for about 7 days before today's startup to move it out and get a good wheel cleaning done. The exterior startup video shows the sound best at 0:19-0:23, it's a very low-pitched, almost "throaty" sound that comes from straight piped cars. It's sort of hard to describe. The sound lasts for about 5 seconds after startup.

I also caught it in a video from inside the cabin. It's a little easier to hear in this one, but it's a very low-pitched, buzzy noise. It seems to go away a little as the RPMs increase. Not sure, though. Seems to be coming from the part where the rear exhaust meets the downpipes. See? I wasn't going crazy, it's a real noise. And no, it's not "new owner overreacting", I would've recognized this noise from before if it was indeed normal. It's not. The video shows the noise best in the very beginning, it's this sort of overpowering very low-pitched noise that you can almost "feel" from within the cabin.

Exterior startup video: New video by Shudhant Gautam

Interior startup video: New video by Shudhant Gautam

vince1 · Dec 22, 2020

I'd check the pulleys on the belt drive system. You could have one beginning to make noise.

Masscomguru · Dec 22, 2020

I don’t hear anything out of the ordinary. To me it sounds like your exhaust just heating up.

08XC90V8 · Dec 22, 2020

Masscomguru said:
I don't hear anything out of the ordinary. To me it sounds like your exhaust just heating up.

Indeed it is hard to hear, but it's definitely out of the ordinary. Did you watch both videos?

The noise is very low-pitched and it's sort of "background" if you will in the videos, but it's really noticeable in the cabin.

08XC90V8 · Dec 22, 2020

vince1 said:
I'd check the pulleys on the belt drive system. You could have one beginning to make noise.

Pulleys are all good, noise is not coming from engine bay.

08XC90V8 · Apr 21, 2021

So this has been bugging me as of late, and it's gotten noticeably worse, as well. Here's the video of it: New video by Shudhant Gautam It's painfully loud and obviously-heard from 0:14 to 0:28.

As you can see in the video, the noise happens just around 1500 RPM, and this video was taken when the engine was cold. It stopped happening when it was warm, and then it started intermittently happening (less loud) today as the engine was warm and I was running errands.

Because it depends on the RPM of the engine, or as far as I can tell, my limited experience is telling me it's the pulleys (?). I'd like to confirm that this is the issue before diving into the pulleys and belt. Which I dread, by the way.

But really, what is this noise? I'll post more videos as I can. And no, I'm not requesting spoonfeeding, I just simply cannot get down to what this noise is.

Monocog007 · Apr 21, 2021

Sounds like a loose heat shield buzzing, have you looked around the engine bay wiggling stuff?

08XC90V8 · Apr 21, 2021

Monocog007 said:
Sounds like a loose heat shield buzzing, have you looked around the engine bay wiggling stuff?

Have not yet but will add it to my list. I know I said I'll take the brakes apart this weekend in my other thread but stuff has been popping up constantly and I'll have to postpone investigating that clicking sound I get from my brakes. I was thinking that it may be a bearing on a pullet? I'm at 70k miles and haven't changed any of those yet. But I'll take a look in the engine bay and underneath for any loose heat shield.

Thanks.

John C · Apr 21, 2021

Can you reproduce it without driving? Should be easy to track down.

ggleavitt · Apr 21, 2021

Follow the @John C lead for sure but I might be inclined to agree with @Monocog007 that it sounds more like a heat shield making the sound. There are a couple shields in the back by the exhaust manifolds and there are a few over the drive-line (which you dropped doing your AOC pump IIRC). See this photo for a couple contributors, the aluminum shield on the left side covers the steering rack and the metal shield on the right protects the pass side axle. I've had to readjust the one on the left a few times after doing work in the area and I've had it off at least once to flatten out the mounting hole and get it more solid with a washer. Can start around there then work your way back and walk the entire drive-line. I would tend to think it's not serp pulleys, at least at this mileage.

Odd Low-Pitched Buzz Noise from Engine

08XC90V8

Masscomguru

08XC90V8

Masscomguru

08XC90V8

Masscomguru

08XC90V8

08XC90V8

08XC90V8

vince1

Masscomguru

08XC90V8

08XC90V8

08XC90V8

Monocog007

08XC90V8

John C

ggleavitt

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