I just went through this on the 1984 300DT. The alternator bearings are sending you a message; either take your alternator to a shop and have new bearings installed or buy a new bosch alternator.

When the warning signs were there, I blew it off and kept driving. WRONG. One day the alternator bearings FROZE UP SOLID, the BELTS howled and smoked like a cat being tortured!!! The car had to be towed home; believe me this was no fun. The towing company DID NOT bring a roll back as requested; it was getting late in the evening so I gave in and let them tow it home. Just call me STUPID and LAZY for not addressing this issue SOONER.

So.......time to replace either those alternator bearings or man up and buy a new bosch. I managed to locate a gently used 65 amp Bosch WITH the mounting bracket from a pick-n-pull for (are you ready?) $50!!!!! It has been working flawlessly since install......