Re: Main and Jib Trailing Edge Flapping
I second FarCry's idea. This was the first thing that came to mind. Every sail (both main and jib) that I've ever seen on 30+ foot boats have leech lines. These are very easy to miss unless you are looking for them.
On some sails, they look like little shoelaces sticking out of the leech of the sail anywhere between 1 and 3 feet above the clew.
On other sails, they could be webbing type straps completely hidden underneath velcro flaps in the same locations as I just mentioned. You cannot see these lines unless you open the velcro flaps.
At any rate, just a little snugging of the leech lines will stop the vibrations in the leech.