Are you noticing bubbles now starting to blow into your pool through the return fitting? Have you noticed the strainer basket (housed in the pump area with a lid on top) is no longer filling up with water like it used to? Have you noticed your skimmer is making a sound like sipping on a Slurpee?
If so, these are all signs that you have a possible air leak or the water level in your pool is too low. With low water level, the skimmer will pull in water and air and cause a noise or air bubbles. Your pump may even lose its prime. Try to maintain your pool water level at least half way up your skimmer.
If the pool does have enough water, then you might be getting an air leak thru the lid on your pump’s strainer basket. That lid has an o-ring. That o-ring needs to be removed and cleaned. Move it all around checking for any cracks or breaks in the o-ring. Replace the o-ring if you find these cracks or tears. Remember every time you open the lid on the strainer basket to clean the basket, you are letting air into your system.
When you put the o-ring back in place, you should coat it with O-Ring Lube (not Vaseline or other petroleum based produces). Vaseline and other petroleum based lubricants will cause the o-ring to deteriorate and break down. You may have a filter with a multiport value. On the side of that value is a bleeder valve designed to release air. Open it slightly to see if you can remove the air and let the water fill up completely in the pump’s strainer basket. Most of the time the problem is low water level in the pool or a worn out o-ring on pump lid.
Other items to check that do not occur as often are; a cracked pump lid, a cracked pump housing, the seal around the pump’s drain plug, a leaking plumbing joint, a crack in the pipe, a crack in the skimmer, etc.