alternator/ vacuum pump, however its not constant or at a steady rpm, so that seems to rule that out. could be another bearing in the belt systems but it does not sound correct when compared to the engine speed itself,
last option, which is starting to sound more likely, is the turbo bearings, if its developed play in the turbo it could be either the bearings themselves making the noise or the turbine/ compressor wheels contacting the housing from the axial play. In my mind if the turbo is spooling unloaded it wont move in its housing very much so it wont be as likely to contact the case, however if the turbo spools under load it will want to shift in its housing causing something to contact, or the bearings to load up enough to cause a noise. also it seems to correlate with the turbine spooling down after it loses pressure from the exhaust, but it only makes noise a second or so after you have released the throttle, and does it in two sort of pulses. I can still hear the turbo whine normally though, which is why I am not so sure about it. It also will make the noise when i apply throttle but below the rpm the turbo spools at.
I hate chasing mystery noises, it is very frustrating, especially because I cannot duplicate it with the van in neutral, so I cant be looking around under the hood really while driving.
any thoughts?
