Conner Fish

I've been trying to look into something and maybe y'all can help. I have a 2014 Chevy Silverado 1500 LTZ with auto temp control. I'll have the air coming from my vents speed set to low and blowing towards me. But then I switch the direction to defrost and then the blower speed automatically jumps to almost high. I have to turn the knob towards high and then to low for the speed to drop back down to normal. I'm almost think the blower motor, what are other people's thoughts?