I got a pair of Mavic Cosmic Elite UST wheels, and I got the same problem. I only hear it when the bike is in the stand. I pulled off the tape and inspected the rim to find what looks like a some sort of counter weight, perhaps to offset the weight of the valve or a metal slug to hold the rim in place during welding (alloy). Couldn't remove it with a pair of needle nose pliers. I was thinking of applying epoxy to it, but it really wasn't worth the hassle. I don't know if this applies to the wheels the OP has, but it's worth taking a look inside.