I have the same bike as Bruce and also use SKS fenders over Schwalbe Winter Marathons. The back fender is great no rattle, the front not as great. I have the same fenders on my 3 season commuter and they are perfect, so I think it is the way they mount to the fork on the Trek. They mount half way up the the fork on the inside which allows the bottom of the fender to move around too much. It is the design of the fork that is a problem, if they mounted at the end, they would be perfect.

I tried Planet Bike fenders on my last winter commuter, before the Trek and they were just too flexible. I think the plastic of the fender isn't stiff and the stays were not as stiff. I would never buy that brand again.