heater blower motor

My 94 chevy has a knocking noise whenever ignition switch is turned on and then after a minute stops until I turn igntion off,then thumps again.Took the blower motor out and checked squrriel cage and housing,but found nothing out of the ordinary.Cage was not out of round and compartment empty.Has anyone had or know what the problem is?Blower still works but is annoying.

That would be your fuel pump believe it or not. My 88 and 96 has done it from day one.