FuZzBoM

Hi all,
Got an odd one. Day to day car is fine. No strange noises etc even when driven hard.
The only odd thing it does is make a knocking noise while reversing up a hill. My friends drive is fairly steep and every time it will knock without fail.. I have checked it and had a google but no symptoms the same as this.
Car has done it since I got it but it worries me if this is a warning for future issues?!?

It is a 56 plate hawk wagon WRX with 60k on the clock. Recently fitted front and rear whiteine rollbars and links etc. so not that. all bushes look to be intact
It is the sort of knock you not only hear but also feel it and it does seem to be car speed related. makes me wonder if the propshaft was the culprit but no signs of it.. Not fully sure if coming from front or rear but sounds more like the front. Any ideas?

thanks,