Knock on the rear axle

Hello everybody, today I noticed a knocking at the HA during slow driving. I tried a little and the following thing occurred to me. – Only to hear in sliding mode and at all speeds. (the faster you drive it the faster “knocks” (at higher speed then a hum) It is not to be heard under load. – Brakes does not make the knocking stop (even if the hand brake is not kicked) – When rolling in “N” also to hear. I mean that the knock In the last few days, new tyres and track plates were mounted on the HA but I suspect that the knocking was there before, but I didn’t notice it. Someone has any idea what it might be?