Does the pitch or volume change with RPM?
If you can get hold of a stethoscope (one from an automotive store with a long probe) it will make it much easier to trace.
I do vibration analysis on aircraft and everytime you try to stop a vibration in one place it will force that energy to another location, sometimes with a different frequency.
It could be timing belt related (bearing) or it could be a crack in something metal( bracket, manifold, etc...)
If you can get hold of a stethoscope (one from an automotive store with a long probe) it will make it much easier to trace.
I do vibration analysis on aircraft and everytime you try to stop a vibration in one place it will force that energy to another location, sometimes with a different frequency.
It could be timing belt related (bearing) or it could be a crack in something metal( bracket, manifold, etc...)