Mix up a solution of food color and water in a spray bottle or use a paint type brush.. Use a lot of coloring or try clothing dye (Rit) but you want it very concentrated.
With the pump running spray or brush dye at various sections of the plumbing that could be leaking. Have someone watch for dye color through clear strainer cover if your system has one. If not you will have to spray an area then look for color as water is returned to pool.
Also listen for leaks and look for dye solution being sucked into leak.
To fix leak try a product called Plumbers Goop. Clean area with acetone apply Goop and let dry overnight.

Could it be you are getting air from the skimmer portion of your system?If the water level is to low it will suck air through the inlet to the skimmer or surface inlet.