If the compressor is starting to fail, it will become noisy when the A/C is on. It's very difficult to know exactly what is happening without seeing your vehicle, but there is a chance that the system is low on refrigerant. When this happens, the compressor will not turn on, and you will not get any cooling from the A/C. Have a shop check to see if the compressor is turning on, and if it isn't, have them check to see if the system is low on refrigerant. If it is low, they will need to determine where the leak is coming from.
I have the same problem. My aircon has just been re-gassed and serviced with filters replaced. the local garage said the pressure was checked and was fine (5?) and that it could be a sensor. They didnt mention anything about a compressor failure. Every time I ask a mercedes garage they just tell me they'll have to put it on diagnostics machine and will cost £100 just for that!