My husbands Kawasaki eliminator is rattling at the gas tank. What is wrong?

It only happens when he reves the engine. He as taken everything off and had friends look at it. Any ideas?

I had a bike that did the same thing. I found out it was the tank vibrating against the frame backbone. The tank mounts were tight but it still rattled. I put some rubber in between the tank and the frame where the noise was coming from.If it's possible have hubby remove the tank but keep the gas lines connected and start the bike and rev it up. If the rattling is gone you know what it is.