When ever I have my A/c on in my 2009 volvo s40 at a medium to low fan setting the compressor will cycle every 3-4 seconds. When the vehicles is in motion and the compressor cuts on it will thump the vehicle (it is a pretty significant thump, enough to annoy me). Any ideas as to why it is cycling like this and why I am feeling that thump? It doesnt behave this way on a high fan setting.

 

Have you checked the freon pressure? a low charge will cause the compressor to cycle. Does it change if you set the climate control to max cold? Is it the thump that is worrysome and seems different to you? If so, I'd try to get a look see - ie with the engine running set the AC on and try to see if there's any odd movement in the compressor or engine when it kicks on or off. If it only happens when the car is in motion, it could be a torque rod or motor mount issue.

 

Just had it charged. Shop said it was a third low but the thump still persists. It doesnt thump when the car is sitting still. Nor does it cycle at max fan setting. The temperature doesnt change the cycling behavior. The dealership told me the motor mount was worn but not enough for them to fix out of their pocket. Fidelity wouldnt cover the cost because its just off the lot.