What could it be!!!

gents.today i did smtg,while driving rangy,and b4 reaching the bump,i put it in neutral and went over the bump to remove any strain from the chain and when i went over the bump at idle and with significant speed the knock was still present...so guess we can eliminate the morse chain as defect,coz there was no strain on it to miss a tooth and rock the shaft back.also when accelerating hard from standing,there is no knock,no machine gun sound,so smooth.nothing slipping.if suspension seems ok,guess there is smtg knackered in the rear diff,coz i have a faint whining from it.maybe smtg is duff in the rear diff although the diff oil is black