I noticed this 2 nights ago, moving on from traffic lights, steady throttle 1st then onto second and had to let go off accelerator pedal but didnt brake pedal just let engine brake, soon as slow down i heard this knocking at the back- like coming from boot or even from underneath. Its rear passengers side. tried it couple of times after. 3rd gear little brake then into 2nd and let it roll -here it comes again. i can do same on 1st- nothing, 3rd nothing. Its not related to road bumps or pot holes, tried rocking the rear wheel- nothing, exhaust moves just slightly but thats normal i suppose. then i checked boot, reserve wheel not moving around, and no tools moving about in wheel well.
No idea what it could be :(