Just today heard a knocking noise. Opened hood and took a look see. Sounded like coming from AC compressor. I shut ac off and knock stopped. It is pretty loud but ac still works. I guess time for a new compressor?
OH, 4.7 engine 2002 ram 1/2 ton 72,000 miles

 

Ouch! Sounds like it. I've never heard of that. I wonder if the load of the compressor is causing an idler/tensioner pulley to make the noise.

 

I went out this morning and did a little looking. Turn ac on and it takes a couple minutes for it to start making the noise. I can't believe it would make that much noise and not tare up and still be cooling. I took belt off and clutch feels good. Turned ac by hand it feels good. Like you I wondered about tensioner since I could see it jumping a little when ac was running but not when ac was off. So I had ac off and turned wheels all way to stop to put load on ps pump belt an all and the tensioner stayed smooth, no jump.

 

The condenser fan went out. Head pressure got up to 350 and it started knocking. Musta been really hard on the compressor. Used leaf blower and blew on condenser and head pressure went right down and knocking quit. Bought new fan motor and all is well now.