Problem with my Integra.

I have a 94 Acura Integra with just over 97000 kms. Just recently I had the time/drive belt replace as per the manufacturers requirements/suggestion. Since that time I'm getting a strong odor coming through the heater vents when I turn on the fan, it smells like used oil. It does appear until the motor has a chance to heat up. I returned the car to the shop that changed the belt, but they could find nothing as did another mechanic that checked the vehicle. The smell is evident when the car is moving, just when I stop with the car idling. Does anyone have any suggestions as to what is causing this problem?