I have a knocking noise on the right side of the engine. It's quite strong at low rpm and, as soon as you rev up, it disappears. We tried compression to see if it was a piston, but the difference between the 8 cylinders is insignificant. All plugs are worn at the same way, same color, no oil anywere. I counted the hits of the noise and I came up with approx 360 knoks per minute with the engine at idle (one every two cycles). As the engine was running we disconnected power to the plugs of one cylinder at the time: no change. We took the cover of the lifters off and, turning the crank manually, we set every single valve to the cose position: no play at all, even pushing down the lifter to see if there is any loss of oil pressure. The rollers that ride the camshaft are ok, they show no play. Thecar has 180,000 Km and burns one quart of oil every 5,000 Km.I have run out of ideas. Can you help me?