Brake Bias Valve?
Or anti-rollbar bushes?
Seems to happen on acceleration, so when the front lifts up slightly and compressed rear!
What happens if you open boot and jump up and down on the back of the car (stationary of course) any noises?
Other than that, is it a rythmic knocking? Perhaps check exhaust rubbers, maybe exhaust is striking the body work, also check your top engine mount for movement, you may have too much engine movement, which adds some weight to the noise coming from the exhaust tapping the body work, that or you have a tax evading Brit' trying to get out of your boot