I have a 1997 C280. The air conditioning whistles (an up and down whistle). It doesn't matter which fan speed I have it set to. It's a very distracting and annoying sound. I recently replaced the cabin filter about a month ago, but this noise just started. I took out the cabin filter and put it back in, but this had no effect. Could the unit be low on freon? What should the pressure read on the low side?
Any help is GREATLY appreciated.