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.

 

Is like a squeak whistle that only lasts about 2 seconds but happens in intervals?

 

If you have one of the vacuum lines going bad, it can cause a whistle. If a vacuum line is going bad, it can cause one of the flaps in the air system ducting to not go into the correct position and thus be partially open which can cause a whistle.

 

Mine's whistled for years, especially after a long road trip. It immediately stops when I turn the AC off. I haven't found the source of the whistle, But I suspect its the blower fan.

 

I was thinking about the blower fan too but ruled that out when he/she indicated that variations in fan speed did not change the whistle.