I have a 1992 Buick Century ( Don’t laugh!)that often makes a clunking sound that I feel as well as hear when I decelerate ( slow down). It happens especially in stop & go traffic. Is it a transmission problem or one of the CV joints?
This is just a shot in the dark, but I think that your problem could be a bad motor mount.
In addition to the lower motor mounts that secure the engine to the chassis, this model also has a “dog bone” motor mount that runs from the engine to the forward bulkhead in the area of the radiator. Those dog bone motor mounts do tend to go bad fairly easily, so I would have that checked out a.s.a.p. as it could be considered to be a safety issue.
CV joints can make a noise like this. A way to test that might work is to slip car into neutral when you hear the noise. If it disappears I would suppect the CV joints.