I have a very loud knocking from im assuming the valve train. When at idle it rattles fairly loud. Rev it up and it gets a bit quieter (possible low oil pressure?) While driving at a steady load it mostly goes away to the normal valve noise. If I lug the engine or accelerate then it get really loud. thought it was engine pinging at first but fuel grade never changed anything. rechecked the valves agin and found the clearance to be 0.018, put back to about .006, still loud. Have adjusted valve 3 times this year so far. Can this be the rockers or valves wearing?