Do you hear the noise when you don't run the AC? If it is the compressor you should only hear it when the clutch on the compressor is engaged (AC is ON).

The only real way to know how much oil is in the system is to take the refrigerant out, open the system, flush it, and put new oil in. If your mechanic put a condenser and dryer on the system last year, unless he doesn't know what he is doing or is not competent he more than likely put oil into the system before he charged it with refrigerant. I'm assuming you are using R134a?

You can add oil to the system either using an oil injector (via the low port) or getting one of those oil charge cans at any auto parts store. Doing this is really a crap shoot as you are not sure how much oil is in the system.

I would ask the mechanic that did the work as to how much oil and what kind he put in the system. Then you will have something to go off of. More than likely he put enough oil in and the compressor is just making noise which may or may not be a sign of future failure.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD