Lately on my 06' Megacab, when I run the the a/c or heater on either of the two highest settings, the blower motor makes a louder than usual "growling" noise. Also when you come to a stop or are going slow the engine has a slight surging. With the blower off or on the two lower settings it runs smooth. Blend door still seems to be be changing "normally", usually some delay between settings, but it has always been like that. Any ideas?