Vibrating Popping Noise under truck

Cannot figure this one out and its driving me crazy. I think this all started when I tried to go through a heavy snowbank and got stuck with snow all the way in the engine compartment and took me a week to get all the ice out. What is happening is when the auto Tranny shiftst for the 3rd time and the rpms come back up around 1600 you feel this vibrating and thumping noise around the frontal centre of the truck. Its worse when the truck is cold. I took off the skid plate and heat shield because it had a crack but still no luck. Does anybody have and ideas!