I've got a '99 Protege 1.8 auto. When it gets warmed up, after a couple of miles, it starts making a kind of high-pitched loud buzzing sound (sounds like it's coming from the right side of the engine compartment). It is loudest around 2000 rpm when accelerating, e.g. after an upshift.

Doesn't buzz when cold.

Any ideas on what this is would be appreciated, thanks!

 

could be the heat shield rattling, my car was doin the same thing, i had a clamp put on the heat shield and the buzz is gone.