My 98 sl2 AC compressor makes a buzzing noise when AC is on only does it at rpms below 1500 at higher revs it is quiet. Blows cold and I can grab the AC clutch and turn it has smooth resistance. Should I just let it go? Or Is it going to fail I cant hear it in the car but when hood is up you can definetly hear it. Let me know what you guys think.thanks

 

Are you sure it is the compressor? Just because the noise starts when the compressor turns on does not mean it is the compressor. The serpentine belt is always turning, when the compressor turns on that increase and changes the load on the serpentine belt. It could be the spring loaded idler that is used to tension that belt. They are prone to failure and when the bearings are on the way out they can make a buzzing sound. That sound will change as to loading and rpm. Buzzing of the A/C compressor is a bit unusual so I would have a tendancy to look for the usual things before the unusual.

 

Well the noise is coming from the compressor I used a stethascope and can hear it in the compressor.

 

The "buzzing" is coming from the electrical coil that energizes and makes the compressor spin. Loose wires inside that coil are usually the problem. Since this is a direct current (battery) type of voltage, this is not normally heard.

One possibility (hope not) is that the alternator has started to go bad, and is not putting out a pure DC voltage. You'd need an oscilloscope to see something like that