A couple of things, the engine is on a metal mount, so the noise you hear is each power stroke being resonated through the fuslage, now that brings up another issue, especially if you have an out of balance prop, fuel foaming. You stated that it runs ok and then the nose up at 45 degrees is gets richer? this might be your problem, when you run it level, the clunk in the tank is picking up airaited fuel possibly, running ok/lean, tilt it up and it should go over lean but now the clunk is in solid non airated fuel, so it goes rich. Just check the fuel line running to the carb as it's running, see if therre are airbubbles running through it, and when you tilt it up they go away is a sure sign of fuel foaming.