Try dropping to a lower gear (or start up hills in a lower gear) and see if the noise quits. If so, that's an indication (but not proof) of knock.

It doesn't seem like knock as he said it's a single thud and coming from closer to the drivers seat itself.

honestly, if you have to go up hill pretty hard for it to make a sound it's probably just pebbles kicking into your rear bumper. and small road imperfections just making it jump about. happens all the time for me if I'm near a dirt road, construction, etc.

edit, just read that it was a thud of some sort.

do you drive an auto?

I remember when I had an auto for my old g35 after some miles transmission started thudding every blue moon under different circumstances.

Okay, the noise still exists and i cannot seem to figure out the issue. It occurs more periodically now and not just uphill.

Here's a better explanation.

If I'm in 2nd, and i have low RPMS and i try and hi it hard, it'll sputter and sound maybe like a misfire coming from the exhaust. Then i feel it under the driver seat. It is like a thump or two and then catches itself and all is well. Anything over 3k -3500rpms and the noise will not do it.

I have less than 40k miles and assure you it is not the engine knocking. I know what that sound is.

Thanks for the help

Used to have something similar on my 911s happen when the motor mounts got week. happened more off the line and into 2nd gear. once car caught up to itself it didnt happen. More load when going from standstill so maybe more load going up hill in the Z. My thud was in the back end but thats where the motor was. if your engine is flexing under that initial load, you might hear the thud - or if it causes things to twist then maybe it is twisting the exhaust pipes?

So, you think I should check my motor mounts first?