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Can you replicate this in your driveway or parking lot?
If so, have a friend stand by, accelerate and then turn off the engine as it is hard to hear over the engine. With the engine off you may be able to pinpoint the origin better.

Has something come loose inside the gas tank?

Take every last item out of the car, trunk and such. Check all wells and behind the seats.

Do you feel the clunk, hear the clunk or both?

I had a clunk in my ERA which I swore was from the front end. Come to find out a pair of pliers was in the rear trunk well, along the side and on a bump it would clunk. Found it one day when I did "Spring Cleaning"... go figger...