best bet... i would think shimmy under the car and inspect the exhaust mounts. grab your exhaust and shake it and see if it is hitting somewhere..(Bet U its just your exhaust

)
i had a knocking sound to the rear of my car to find out i had dropped a rear rubber mount on the exhaust and it was banging on the spare tyre well and Panhard Rod.
while your there check your Trans Mount Bolts are tight so put a socket on them and nip em up as well as the centre bearing mount. Give your Tailshaft a shake as well
Goodluck man let us know..
P.S: if your car is hard to get under park it on the gutter(2 Passanger wheels up on gutter, drivers side wheels on road and slide under it... or use your jack and suitabe stands or your spare tyre to shove under the car, Never rely just on the jack.