Air getting into the system points to a suction side air leak. I'll recycle my usual response. You might also try using the search box for suction side air leaks.
Round up the usual suspects: low water level, stuck weir (flapper) in the skimmer, loose drain plugs on the pump, poor seal on the pump strainer basket, and leaks at the fittings, especially any couplings.The glass bulb on the filter is a sight glass. It should show empty unless you're backwashing, when you use it to see when the water is finally clear. When the pump is on, do you see or hear a vortex in the skimmer? Does the flap swing free? The O-ring on top of the pump basket is a really common culprit, it needs to be clean, lubricated, and have no cracks.