Cartridge Filter Goes 'Glug' 'Glug'

[TOMMCQ]

I have a Pentair Clean & Clear cartridge filter that makes a bubble noise when the pump is turned off.
When I turn the air release valve it lets out the air along with the water in the filter tank and that stops the noise.

I have a PebbleTec finish and the pebbles get into the tank. I wash out the pebbles through the 1-1/2" drain hole in the side of the tank, but I'm wondering if the other pebbles get washed into the inlet/outlet pipes under the tank and that is creating the issue.

Do you have any thoughts on what would cause the bubbling and how to eliminate it?

Thanks for reading my question.

[Teapot]

It sounds like an air leak, isn't there a big gasket/O ring around the joint? I would look there first.