I've noticed a strange noise on my 2005 XC70. It seems to be a metallic click or clunk at 15mph. This occurs the first time the car reaches 15 mph after starting the car and engaging Drive. After that I don't think it happens again, but I need to drive it some more to know for sure. It doesn't seem to be related to transmission shifting either.

Does this sound familiar to anyone? What could it be?