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The lift pump is an interesting little machine. The PTO shaft makes it turn, so if you disengage the PTO, you gots no lift. The pump uses pistons to pump the fluid to the lift, sorta like an air compressor. The only difference is, there are 4 of them pistons! There are two blocks (front and back) and each block has two pistons on it, one on each side like a horizontally opposed 4 cylinder engine if you know what i'm talking about. The blocks with the pistons are cast iron. There is a cam that slides over the PTO shaft to make the piston blocks go back and forth and pump. It is brass, i beleive. The cam doesn't directly contact the piston blocks, though. There is a second brass block that fits inside the piston block and that is turned by the cam. Since brass is relatively soft, it will wear down faster than the cast iron piston blocks. When the sides of the brass block get worn out, they smack back and forth inside the cast iron block sorta like piston knock. That is the knock you are hearing. You can get a rebuild kit for the hydraulics ($280) or you may be able to get just the parts you need separately. The rebuild kit comes with new piston blocks that you may or may not need. It does NOT come with a new piston cylinder for the lift or piston itself. We recommend that you go ahead and replace the piston in the lift cover to the NAA style with rubber o-ring and leather back up washer for about $27 total. Again, not sure if you can get the cam blocks separately or if you have to get the kit. Go ahead and get a new exhaust/relief valve while you're in there. Now you know what causes that knocking. If you have an F0-4 manual, look for the exploded view of the pump and you will see the blocks i'm talking about. Jeb
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