Noisy heat pipes

Plumbed 2nd floor heat lines. When furnace kicks in the pipes "tick" and "creak". Is it do to air in the lines or is there another reason? Should I bleed the line?

Note I'm not a pro. I assume that this is a hot water system. Many creaks and pops are the pipe rubbing on things - the holes through walls and floors need to be done with room for the normal expansion of the metal piping. Same is true of the radiators as well. Air usually doesn't creak and pop, although it does expand much more than the water does, and should be eliminated. WHen filled, there is always some disolved air in the water. Eventually, it often separates. The goal should be to elinimate all air in the system.