Knocking noise on a 1996 Jimmy

1996 gmc jimmy knocking at medium RPM no knocking at idle or high RPM it sounds like it may be coming from the transmission any help would be appreciated.

See if it does it in park when you rev the rpm's

Yes it does make knocking at medium RPM in park, drive and reverse. not at idle or high rpm
the sounds seems to becoming from the back of the engine or maybe transmission it's hard to tell