My bad... i didnt read your post properly.... sorry.
Revised answer is,,,, Yes, you could have a defective spider gasket in your multiport valve
Or as i said before, the particals could be extremely small and be passing straight through your filter media.
What you could try if your confident enough is dismantle the top of the multiport valve and lube the gasket with some magic lube or something similar,,, then put back together and see if for a short while it stops,,,, if it does then you know its the gasket.
Hope this helps.
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