Hi,

​​​​​​I Have a 2001 volvo s40 2.0T.
​​​​​​I took it in for repairs and now I hear a high pitch sound coming from the engine bay when the car is under higher boost / revs.

I checked the exhaust manifold bolts and all is fine.
Can this be because of a vacuum leak , reason I am asking, i tried the test with smoke from the vacuum pipes and smoke is coming from below the intake manifold. ( Please see picture attached of the pipe I blew the smoke into ).
Note: the High pitch noise do get a bit softer when the temperature increase

 

I would see where the smoke is coming from. There's vacuum lines under there and the pcv system too.

 

pull the dipstick to see if the noise goes away - if so, start investigating the PCV system...