[QUOTE=Benzadmiral]Many thanks to all who posted. I stopped by my usual independent, who worked on my venerable 420SEL for the last 3 years. He lifted the hood and inspected, listened to it with A/C on, off, then on again, and pronounced the dread sentence. "Yeah, compressor's going."QUOTE]

You may be able to save it by adding some oil to the system. Freon leaks are always accompanied by oil leaking out of the system as well. Since you don't know how many times freon has been added, without oil, it may be dieing for lack of oil.

Don't overlook a noisy serpentine belt tensioner. They have a knocking sound that may be relieved by turning off the AC, which is removing some load from the belt.

Good luck,

Jim in Phoenix