Sounds like the dreaded second gear problem that these bikes are renowned for, though it also happens to first gear on occasions (ask Bryan). What has happened is that the engagement dogs on the backside of the gears round off due to a very small contact area, resulting in the machine gun like rattle that you can hear (and feel!) when you accelerate. Sometimes this can be cured by attacking the dogs with a dremel and back cutting or undercutting them so they have a slightly angled face so when they mesh together, they actually pull themselves tighter together, but in bad cases, the dogs are that badly damaged that the gears have to be replaced. Unfortunately, it sounds like yours might be really bad.
Does it happen all the time, or only if you give it the gas off the line?
