Air Conditioner Hissss and Rattle?

Does anyone have an idea why the following is occuring on my 1991 300E?

1.) The compressor cycles on and off every 12 to 18 seconds while the car is cruising between 60-70mph on the freeway. I can hear the "hissssss" of the compressor kicking on for about 6 seconds, then turning off for about 12 to 18 seconds, before it cycles back on again with the same "hissssssss." Is that normal? It used to just cycle on every few minutes, not every few 12 seconds. When the car is idling, though, it takes longer between cycling on and off - a few minutes - like it used to do before. It seems to be dependent on the speed the car is going.

2.) When the car is in idle, and I listen carefully to the engine area when the compressor comes on, it makes a loud metallic rattle - like a rapid stacatto metallic "rat-tat-tat-tat" but that stops as soon as the compressor turns off. This is accompanied by a slight whistle, or squeeky wheel-turning sound. Of course, the noise starts back up as soon as the compressor kicks back on.


The a/c is still running very cold, but the noises are new and seem to be getting louder as the days go by...

What are these telltale signs of??? I'm afraid to ask.....

[aqua190]

sounds like you are going to need a compressor soon.

[jcyuhn]

How certain are you that the compressor is cycling on and off at highway speed? The hiss and moan you are hearing is the expansion valve admitting more or less liquid refrigerant into the evaporator. It's a typical sound on a 124 which is a bit low on freon.

The rattling compressor means you are likely going to buy a compressor pretty soon. Try having an a/c shop add some refrigerant oil to the system to quiet it down. They can do this without discharging the system. It may buy you some time. Not running the a/c until you get it serviced might be a good idea too. (Yeah, I know how hot it is in Ft. Worth...)

- JimY