Engine starts easily, with a couple of seconds of tapping before it runs quietly once oil pressure has risen to 45 PSI (by an SW gauge I've connected to the port next to the oil filter). After a couple of miles the oil pressure has decreased to single digits and the tapping sound returns, becoming intense. After shut off and cooling this thermodynamic cycle repeats. The spark plug at front left, under the tube to the air intake, was oil-fouled. I believe there is a faulty engine component which expands with heat, releasing oil through a cracked metal part. It sounds like in the top of the engine. Where should I begin to look?