If your vehicle is showing loud clunk or thud from underneath when letting off the accelerator, here are the most likely causes and the parts you'll need to fix each one.
U-joints connect the driveshaft to the transmission and differential. When they wear out, the resulting slack causes a loud clunk when driveline torque reverses, such as when letting off the accelerator.
These rubber mounts secure the powertrain to the frame. If torn, the engine or transmission can lift under acceleration and slam back down heavily when you lift off the pedal.
On front-wheel or all-wheel drive vehicles, worn inner CV joints develop excessive rotational play. This slack creates a thud when transitioning from acceleration to deceleration.
Worn rubber suspension bushings allow the control arms to shift forward and backward when the driveline load changes, creating a clunking noise from the wheel area.
If suspension components, mounts, or driveshaft bolts were recently replaced and not torqued to specification, the shifting parts will clunk loudly under load changes.
On vehicles with two-piece driveshafts, a torn rubber center support bearing allows the driveshaft to bounce and hit the undercarriage when torque is released.
Internal wear in the differential gears or carrier bearings creates excessive slack. This slack is violently taken up when transitioning between accelerating and coasting.