not to general a question, as long as you don't mind a general answer!
Air in filter is usually a suction side leak... when you turn the pump off, it may drip and show you where.
A common place is the filter basket at the pump- the lid sometimes is finicky. Is this basket full, or does it have air in it?
I've also seen people use incense to look for leaks- on a calm day, light the incense and move it along the piping (with pump on) and watch the smoke...
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