[John C]

I've also started to notice a whining or a faint whistling sound that follows the revs.
I'm thinking there a a boost or air leak or maybe something's cracked

I don't know squat about Volvo Diesels, but know quite a bit about Diesels in general. Black smoke is too much fuel and not enough air. The first thing to do is to check the air filter. The faint whistling sound does indicate a leak of some sort and/or a failing turbocharger. Check all your intake piping for leans, and check the impeller of the turbo for excess radial play, or evidence it is touching the housing.

 

[John C]

a little oil on the intercooler pipe that runs under the engine,

Can you tell where it's coming from? Those pipes are under pressure when the engine is boosted.