You're right, I do.
But you will have to take the transmission apart to fix it.
I've only had it happen once, but sometimes the little piece in there slips out of where it is supposed to be.
(I know how to fix them, but being self-taught, have no clue what the parts are called)
Right smack in the middle of the transmission is this little piece that looks like a sideways "U", alright ? - now that piece is somehow integral to pedalling the bike, don't ask me how - but it is, if it slips out of where it is supposed to stop, you can start the bike ok, but when you try to peddle, all you get is a rattling sound.
As to how to fix it other than taking the transmission apart and putting it back together ? dunno - also, this is more rare, but possible....there is a little drive chain inside the trans that is part of the pedalling assembly to, and it might be broken, still, it looks like you will wind up taking the tranny apart either way.
Get an oil-catch pan, and a quart of 10w30, and the paper seal-gaskets for the transmission seals BEFORE you take it apart, too - because if you are going to disassemble it, best to use new fluid (the 10w30) than put the old stuff back in there, especially if due to this problem it has metal bits floating in it.
And when you take it apart, you can forget re-using the seal gaskets currently in place, it just flat don't work.
Good luck on that one, the whole transmission is actually damn simple once you take a good look at it, and using a little logic, and good preparation, it shouldn't be too hard to fix this.
As for the gasket, beer can, hahaha - that's a neat idea !
Normally the A3 doesn't have gaskets, it's supposed to be a metal-to-metal seal, but since it's two different metals, over time it doesn't seem to work too well in practice (looked good on paper, I guess).
For mine, I went to the local hobby store and bought some sheet material used for shims, and luckily two different types (found out the hard way that brass doesn't work) but I suggest you simply buy aluminum to begin with.
Since it's cheap, buy a couple different thicknesses, just in case...you may have to play by ear to get that one to work out.
I hope all that helps some,