I was chasing down a rattling sound I could hear in my garage when I would rev up the engine on HD Motortrike Trog kit. I thought it was my exhaust or the baffles inside the mufflers. Turns out it was the brake shoes inside the rear brake drums vibrating and making the metallic sound I was hearing.

I had the rear brake applied while the engine was rev up and the sound was not present

So if you hear a metal rattling sound when you rev up your engine, put your ear down to rear wheel. Its possible its your brake shoes inside the rear drums.