I had an annoying whine that developed in my 35d and it was a bad alternator bearing.

Symptom: engine whine, increased with engine speed, whether in Neutral or Drive. Also, any other failed bearings in the belt pulleys can make the same noise. Just get your assessed and go from there.

I replaced it with a new alternator, although I probably could have found a reputable shop to rebuild it but decided to just go with new.