Sounds like your ball joints have gone. Do you notice more or less vibration in a turn left or right? Also if they installed an aftermarket heater core it may not be functioning properly and will not allow the recirulation door to fully close. Or it could be that when the heater core was changed out the mechanic failed to do a thorough back flush of the system before installing the new core and thus filled it with rust and debris...Recommend find another person who wants the headache more than you.