I have a knock/rattle which seems with great certainty to be coming from my alternator, based on using a screwdriver as a stethoscope and feeling the knock when touching the the alternator. It's been getting worse for some time, and at first I thought it might be a piston wrist pin or a rod, but the noise vibration are stongest right on the alternator. At interstate speeds, which is 95% of my driving, I really can't hear it at all and even around town its barely audible; mostly audible at idle and lower rpms. I've never had this phenomenon before in my life, but then I've never had a car go 328,000 miles without any problems with the bolt on engine accessories except the starter which failed at 308,000.

Has anyone else ever experienced a knocking/rattling alternator ? No other indications are present except the knocking.

 

I'd take the belt off and spin the alternator by hand. Is it quiet? Is there any play in the bearings?

A really tinny rattling from the center firewall area is usually just the heat shield on the catalytic converter and / or exhaust pipe.