PackardV8 wrote:Very strange - the noise is load-activated at certain RPMs but doesn't change pitch/frequencey with engine speed. Therefore, it almost can't be a rotating assembly, such as the flywheel or damper.
Zwede, here's some more random guessing and thinking out loud. Packard's comment brings up motor mounts, rear trans mount and also the differential mounting. Sorry for suggesting the motor mounts after you ruled them out but are you sure about the motor mounts? Weird things happen when the motor moves, lines and hose get tugged, maybe something around the radiator and electric fan? Apologies again but a good way to check the motor mounting is the leave the hood up, parking brake on and let the clutch out trying to load the motor as you peer at the engine looking for movement.