Like your noggin getting smacked upside the wall

 

That's drivetrain slop. it's normal. I feel it on my 04 if im not gentle on the gas
while increasing throttle angle exponentially. I feel a hell of a lot more on my Cooper-S.
It's the torque from the motor pushing against the motor and tranny mounts to
propel the car forward.

 

if you are at 20mph and stomping on the gas in 2nd gear, i'm betting your tires are slipping. the clunk you're feeling is probably the VDC kicking in. try turning off VDC and see if you still feel the clunk.

 

I dont see how this could be. I believe its stored power finally emerging. this shouldnt be an issue, but I wonder how your motor mounts are holding up???

 

I can't help but wonder if there's something loose in the spare-tire well. It's worth taking a look, and making sure everything is really secure in there, before taking it to a dealer. My wife once told me she needed to take a car in for a loose seat mount because she said it was clunking around turns. I looked under the seat and found a bottle of water.

Why did I look under the seat? LOL About 15 years ago I had a serviceman hand me a baseball from my spare-tire well after I took my car in for a knocking sound from the rearend. Nothing says "I'm a dumbass" like that look on your face when they hand you that 'mystery' noise and it's nothing remotely automotive. At least they didn't charge me! Strangest thing happened; a week later the sound showed back on the way home from baseball practice.

 

Are you sure it is not engine knock? Are you putting in 91/93 octane?

 

I did read before i posted and I maintain that 2K is not even close to the powerband on the G. Look at the torque curve and you will see for yourself. I can't imagine "punching the throttle" at 2k in second gear and expecting the car to pull at all.

 

my exhaust is to loud to hear anything... but nothing i notice.