I bought a 99 Merc 90 ELPTO for my pontoon that the previous owner said was blown. I checked and it had 125 psi on all three cylinders so I figured it can't be that bad. After installing it I ran it on the hose and you can hear a knocking sound that seems to be for the midsection. When I put my ear up to the midsection it's loud. Before I drop the lower unit and try to run the engine pressing the hose up to the tube I was wondering if anyone had any suggestions. I guess I need to figure out if it's the powerhead or lower unit that needs work. Is there anything in the midsection that could cause the knocking? Thanks