Why have you got rear wheel knocking noise after bearings changed?

It could be several things, your bearings could be under a pre load, which means there is exesive pressure on them caused by them being over tightend, it could also be as simple as a brake hanging up on the drum or disk. you may have a bent rim possibly but that would cause the car to vibrate. Also you may have a worn spindal, the shaft that the bearings slide onto. You could possibly have a brearing catching or binding up. I would definitely have it looked at so you don't have a bearing go out because it could mean you lose you tire completely.