Normal. Your fans are always on when your Mac is on, spinning at a minimum of 2000 rpm (for MBPs) or 1800 rpm (for MBAs, MBs and minis). They will spin faster as needed to keep temps at a safe level. If they're spinning up without increased heat, try resetting the SMC. PRAM/NVRAM has nothing to do with these issues, so resetting it will not help. Also, make sure you don't block the vents, which are located at the rear, near the hinge.