There's a single "click" or "clunk" noise that appears between accelerating and decelerating. Does not matter how fast i drive or what RPM the engine has. For example, if i start driving then i'll hear the single clunk noise and if i release the gas so it will decelerate or break then i'll hear the exact same clunk noise.
Had a mechanic that my family knows have a look at it. He changed the ball joints since they were too loose. But, the sound persists. He checked all he could and thinks it's the transmission / powertrain. I read on this forum that these symptoms might be caused by the engine mount. I put on the hand break on the car and stepped on the gas a few times to move the engine, but there's no click unless the car is moving.

I recently bought this car. A 2006 model with automatic transmission that has gone 107 000 km / 64 500 miles. Am I doomed ?

Here's a video of the problem. You can hear the click at 2 seconds, 13 seconds and until the end.