It's Fred when he finds the key to the sheep barn. But seriously we had an engine do that and drove a bunch of us nuts,turned out to be a bad PVC valve and the engine was trying to vent it self at the rear main seal.Try taking the breathers or PVC valve off and letting the engine vent at the valve covers.If you think it's the belt remove it and listen for the squeel. I assume you check for anything rubbing(mechanical)