I've had Ford replace my belt for the same reason. They first replaced it just before warranty expired, and then have replaced it 5 times since (every time for free) for the next 3 years. The problem goes away for a couple of months and then comes back. Problem completely disapears in winter (cold up here in Canada eh !).
I've mentioned to them that they should look at the tensioners, but they keep insisting that the last belt they gave me was faulty. Ah well, I'll bring it back in for my free spring belt in the spring ....
