I recently replaced the heads and intake on my 400m. Everything seemed right, and it started on the first crank. Running down the road all guages look fine, but when I stop after a 15 minute or longer drive the engine coolent thumps. I don't know how else to describe it. At first I thought the engine was still running so I got out and opened the hood and I could watch the upper radiator hose jump with each thump. It went on for a few minutes about as fast as it would take you to read thump, thump, thump, thump. No loss of fluid but I did notice when I turned the ignition back on the temp guage was pegged. As soon as I started the engine it dropped back down to normal. The engine and radiator do not feel unusually hot. I have no water in the oil, no apparent loss of coolant, just this thump thing. Anyone ever heard of this?