Probably your laptop vents are blocked when you place it on your lap. The fan noise can be due to dust accumulation or due to unnecessay process hogging up your resources. Check for the useless apps running in the background and stop it. Make sure HP CoolSense is running.
Keep your BIOS updated(use HP Support Assistant). Also update your windows. Check for viruses and unnecessary software that may have been installed. If this doesn't solve the problem try resetting your system or ask the HP service centre to check for problems related to dust or fan.