I've been sorting out various issues with my engine and now I've discovered that when the engine is under load the valves sounds like it's a diesel. The engine was recently rebuilt including a new oil pump. I put in an aftermarket oil pressure gauge and its very steady at 40 psig. The valves chatter only when I'm going up hill, which I do a lot here in Pittsburgh! So what's causing the noise? The oil level is correct too.