Petia

I just had this problem.
First I tried to fix it in a quick and dirty way by forcing small bits of (music) paper in the seam. It fixed the rattling but damped too much.
From this experience I concluded that glue or solder are probably the worst solution: it will likely dampen the mute too much.
I used pliers like others recommended, gently applying pressure to several points and playtesting after each squeeze. This works great, just don't overdo it, as commakozzi mentioned. It's a one way road...