Hi All! Long time reader first time poster. I have been a GMT400 enthusiast for years and have one that is completely stumped me. I have put a new clutch and rebuilt the engine on my 1988 K1500 replaced the u-joints. I drained and filled the differential gears looked good and replaced brakes and drums due to wheel cylinder leaking and replaced seal on axle right rear side. After getting through testing and tuning I have a noise from rear of truck that I can feel in seat and transmission shifter. Here are symptoms as follows. In forward gears I feel and hear a noise that best described as whomp whomp at take off. Maybe even a binding feeling slightly. At first I thought I had left bolts loose on transmission to engine. Torqued those. Went through all bolts under truck and made sure that nothing was loose. changed transmission oil. (shifts better) shook the driveline and regreased u-joints no change. I thought it was like the springs loading up and rubbing together but can't seem to see anything that is the smoking gun. Any ideas would be awesome.