So i recently i replaced the water pump, thermostat, and the intake manifold gaskets on my c4 that i purchased, it was put up in a barn so of course a lot of connectors and hoses had to be replaced in the process of putting it back together including a few sensors that broke from being too brittle, and also including the ac compressor which decided it wanted to start making a high pitch grinding sound during one of the start ups. after finally getting to take it out and after having a good test drive when i got it back to my house i noticed a knocking noise which sounded like it was coming from the front of the motor. i took the belt off and fired it and fortunately the sound went away meaning its coming from another one of the accessories. i spun all of the pulleys and noticed the air pump has a slight wobble which does make a wobbly noise when i spin it freely, but the noise sounds like it may be coming from the down by the power steering pump (which was bone dry before i filled it). my gut tells me that it should be the air pump and my ears are playing tricks. i figured i would post on here and see if i should go ahead and replace the air pump.

 

Welcome to the forum. Does the air pump have an exposed nut in the center of the pulley that you could chuck a socket in a drill and spin the air pump? It might take a right angle drive attachment due to clearance problems.