I am in denial but I think I am in trouble. I just turned over 345 trouble free hours on my 912ULS when I had a new noise last week. Kind of a slight rattling noise that comes and goes. Could not find anything loose and the engine ran fine at all speeds. Today I ran it without the cowl off and hear a definite but slight rattling,knocking sound at 2150 rpm on warm up. It goes away at 2000 and 2200 and on up. I am still flying but since I don't like off airport landings, I have to ask if anyone has any ideas.
....Please say its OK...
Oil usage is nill and has been changed on schedule since new. Average fuel consumption is 3.7 and always non-ethanol 91 octane.
The cost and loss of flying time is going to be grim. I don't even know where to get it fixed if I have to.

thanks in advance
Rich L