One time I heard a noise similar to what you're describing as I was leaving my home for a ride. As I accelerated the noise got louder, and as I decell'd, it got quieter. As the ride progressed, the noise got increasingly LOUDER! I said fuck it, if it blows, it blows and pressed on. I would have bet ANY amount of money it was valve train noise, I was convinced that I'd be doing a tear down in the very near future.
I made it home and started inspecting the bike, then it hit me. About a week prior I had the tank off.....it was the rear tank mount. Yep, all that noise and clatter was from a loose tank mount.