Air in filter/ DE in pool

I have a half new half old Pentair SMBW 4060 filter. When the pump is running there are no visible leaks. When pump shuts off you can here air coming into filter. It's a slow sputter sound. Slow enough where DE will come back into pool but pump never loses prime.
When pump comes on in the morning of coures there is a big gurgle of air that comes into pool with sometimes a trace of DE through the retuns. Once air purges there is no noticable bubbles through returns.

Have had the bottom of filter, a few grids, and the gasket changed. Any ideas?
 
Welcome! :wave:

I get that sometimes and it's from the air bleed on top. Sometimes I check it and it doesn't want to close right and sometimes it's smooth as can be. Things deform under pressure so I suspect something gets sealed tighter by pressure and then starts sucking air when it's off. Stretch a balloon over the air outlet and see if it gets sucked in overnight.
 
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