Excessive engine noise - any idea what it is?

[MikeT]

My Canada 1 has had a bit of a minor buzzing whine on the over-run that at first I attributed to drivetrain wear, when I first noticed it about 3 years ago.

 

Now that it's getting louder, I have listened more carefully and it is not a drivetrain noise. The noise is directly proportional to engine RPM and in fact it happens when the car's in neutral and it's revved to 2000+ RPM.

 

It sounds a bit like a loud mechanical supercharger, sort of a buzzy lower frequency whine.

 

Engine performance is not affected at all and it still pulls as hard as ever up the local "benchmark hill".

 

My first though is a failed diode in the alternator, because they can get noisy in such a case but the charging is fine. It could also be a bad alternator bearing. But it's a newer unit from around 2017....so maybe an unlikely source.

 

It could also be the vacuum pump or water pump....but anything else?

 

Has anyone had a symptom like this before and found the cause? Thanks 

[Mjolinor]

First guess would be alternator bearing. Take the fan belt off and see if the noise goes away.