Robb2175
Sounds like its clogged at the diaphragm. Just install a new fill valve. Easiest way to fix. 7.00 at Lowes.

Nicholas
I don't think it is air, I think you are hearing the high-pressure hiss of a very small volume of water squeezing by a blockage (sediment/rust/scale/etc). When you turn off the water (no pressure) it lets the sediment drop away from the restriction due to gravity. When you turn it back on, water flows until the sediment is flushed back into the restriction (the fill valve). Pull the valve out, clean it and flush out the water line out and reinstall. If this fails to solve the problem, replace the fill valve.