Could be any number of bushes, I had a particularly awkward one to find that sounded like it was coming from the fuel tank area, after getting a buddy to jump up and down on the tow bar while I crawled around underneath we traced it to the panhard rod bushes at the fornt, the sound was travelling right through the chassis and appearing to be at the rear end.
Another one turned out to be shock absorber bushes, took bloody ages to find the noise but once there it was a reasonably easy fix. From your description I'm erring towards shock absorber bushes.
I thoroughly recommend the use of an able weighty assistant to "bounce" vehicle whilst you crawl around underneath trying to narrow down knocking sounds.
Now if said sounds only occurs under power and not when stationary and under able weighty assistant conditions then you need to start looking at the drive train starting with the obvious bits first such as UJ's on the propshafts etc.
Happy hunting.
Muddy