1994 Honda Accord sometimes a thunk in reverse

I took my little 1994 150K accord to the honda dealer today (I might add they did not try to sell me a transmission). They ran tests, changed my transmission fluid, and said I had a minor oil leak to keep an eye on. The pressure of the tranny was ok. No other issues cropped up. I forgot to tell them about the THUNK I hear at times when I am in reverse. If I am backing slowly, it will not thunk, but if it is faster, it will thunk after about 6 ft. This isn’t an issue unless the thunk is a forewarning. What do you guys think?

And it is an automatic…

I would suggest that you have the motor mounts and the transmission mounts checked. A broken mount or two will produce these symptoms.