I am having an issue with air bubbles in my pump basket. When I turn my pump on it runs fine with almost no bubbles. After 30 seconds (sometimes a couple minutes) slot of small bubbles start to form and I hear a gravel/rock sound in the pipe coming out of the ground. The poly pipe underground is connected to a 45 elbow. I only have one pipe from my skimmer. I also hear bubbling in the filter.
What could cause the cavitation/rock/gravel sound in the pipe? I don't seem to be losing water so I'm not sure if there's a leak underground.