Rear brakes seam to grip when i am parking my ford f150?

i had my rear brakes fixed 3 months ago changed calipers and pads but when i am parking and putting the truck in gear it sounds like a grip then release. is this normal or can it be that the rotors or brake pads are dirty. frod f150 V-6 year 2003 does not make any sound when i am driving only when i park.