i just bought a 99 dodge ram 2500 4x4 v10 it was running great untill about a month ago i noticed knocking noise comming from underneath. Took it to the shop and they put in a new fly wheel ( old one was cracked). Now i only notice the noise when the truck is shifting gears (20-30 mph) noise is consistent with rpms. and then its gone. also the noise is there when im pulling a trailer ( out ot od) at about 60 mph. Its annoying. Someone told me it could be my air pump, but im wonderind if its not something else. Please help?