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Discussion Starter · #1 · (Edited)
Rattle/buzz at cold start - anyone else have this? S40 2.4i

Hey guys,

I've been having a buzzing, metallic rattly cold start for as long as I can remember. The noise goes away within ~30 seconds. I figured it may have had to do with the nearly disintegrated flex pipe I had, but with a new one of those as an assembly with the catalytic converter, the noise is still there. I tested just about the whole system for leaks using the ol' shopvac up the tailpipe and soapy water method, and nothing to report. Nothing seems to be hitting the exhaust either. Anyone else have this or any tips on where to look?

Some video of the issue: https://vimeo.com/345936535

Thanks!
 

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Sounds fairly normal to me. Our S40 2.4 sounds similar.
 

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2007 Volvo V50 T5 AWD M66
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Possible loose or broken manifold studs. The manifold gasket can make some sound when it's cold.
 

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Discussion Starter · #5 · (Edited)
Gave it a listen - definitely from the fuel rail area, not behind the engine, so I think I lucked out on the exhaust manifold front.

If it's the injectors, anyone know what makes them so damn loud? My last 2.4i definitely wasn't this noisy.
 

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I personally think it's related to cold metal (wide tolerances in design) on cold metal for the injector and the plungers. after they fire for a while cold they heat up enough to close those tolerances either from the electrical energy passing through them or from the constant pressure and friction of the fluid being pushed through the strainer - further helped by the heat of the enginebay. There's a big difference in tolerances from 60*F ambient and 200*F when warm on micro electronics subject to pressure of the fuel pump.
 

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Discussion Starter · #7 ·
A quick update - a fuel injector cleaner additive makes a noticeable dent in the noise, but only temporarily. The injector theory seems likely
 
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