I had a vacuum leak that was throwing codes P0171 and P0174, so yesterday I changed the IMT o-rings, the PCV hose, and the brake booster hose. That fixed the vacuum leak, but now I have a low pitched whistle. It only happens once or twice when I first start driving, then goes away. It is not a whine, nor wind noise. It is short duration, maybe 2 seconds, and the pitch is about like the whistle you get when you blow across the top of an empty soda bottle, but somewhat louder. The car is a 2002 X-type 2.5L with auto transmission. Any ideas?

 

drive shaft center bearing? Does it happen when slowing down or when engine braking.

 

It started immediately after I fixed the vacuum leak. Now, however, after driving it around for a day, it stopped. So I guess that some part of the vacuum system was not seated properly. Perhaps the brake booster valve needed to "wear in." Anyway, it seems to be done, so I am not going to worry about it unless it starts again.

 

Update: it still happens occasionally, when the engine has been stopped for a while. It is just a short whistle, and then it stops. It is not too annoying, so I am going to ignore it unless it gets worse.