I have noticed a weird thumping noise coming from under the passenger side of my truck for awhile now and its starting to bug me. It does it at any time and seems to be more often with the heater on. Is the cooling fan or somthing of that nature? I dont have a clue. Its sounds like a pressurized thump only sitting a light or warming up. But sometimes it does the noise and other times it doesnt. Please help its driving me crazy! thanks

 

I think that's the A/C compressor cycling. Mind does the same thing. It sounds like someone hitting the firewall with a rubber mallet. My dealer says that it's normal on the new compressors.

 

It is the Air Conditiong Compressor, mr176 is right.

 

Thanks alot! I just thought that was very weird and annoying. I was thinking it had something to do with the heater because I didnt notice it during the summer months. I guess I will have to get used to it.