944T4ME

18K on the clutch

special tool

Jack up the rear of the car, put it into first and see about how many degrees of rotation the wheel will spin.

JimV8

In the rear jacked up test how tight should it be if the clutch is OK.

car_slave

I vote for parking brake. A miss adjusted or loose one will make a very load knock, or knocking, every time you hit the brakes or move from a stop. Go to Clarks-Garage check the proper adjustment specs. Also try useing the parking while driving (no more than 20mph) and see if that sounds bad.

Other than that, maybe a bad transmission mount. Give it a shake.

StoogeMoe

Could be the rear shocks or sway bar. You could check that by giving it the old knee on the bumper.

brad-cam

While it might be a retarded woodpecker, I would give a strong vote for the rear brake pads. I recently changed mine to Mintex, and to my surprise, the first time I used them pulling out of my driveway, I heard a loud knocking noise. Turns out the pads are loose in the calipers and they slam into position. It's quite loud. I put some of that blue brake pad goop on the back of the pads and it fixed it up. Your mileage may vary...

Luis de Prat

If you fit the stock noise reducers on the new pads, this clunking should not occur. The noise reducers stick to the new pads and hold them in place to eliminate any vibration, e.g. "squealing, clunking," etc.