Hey guys I have a 2003 Infiniti g35. There is this loud clunk noise that happens under the car either when i accelerate or brake slowly. Sounds like it could be coming from the transmission. I’m not sure. What do you guys think it is ? It’s not a rattle, but more so like a loud clunk.

 

Inspect the rear differential bushing, that's a common failure point and folks typically replace it with a polyurethane bushing for longevity and better feel.

 

it usually only happens like every so often if i accelerate or brake while slowing down.

 

Look underneath the rear of the car, the oem subframe bushings are viscous, and when they fail the fluid will leak onto the subframe.
I would check all the engine and transmission mounts too.