We have a 2001 Windstar, about 8 months ago we had the P0171/P0174 fix done. Lately the engine seems to be running rough. A few days ago we turned the heat on after driving about 40 miles and only cold air came out. After turning on the rear heat the front heat kicked in and has been fine ever since. It sounds like a low constant air sound even with the blower shut off. Any ideas?