Could be some small air bubbles trapped in the plumbing. If it's a top mount valve it's quite likely. I've never seen one but I've read there is an air bleed valve on some filters. My Hayward does it sometimes and eventually quiets down. I wouldn't recommend it but I've sometimes pushed on the valve handle just a little to open up all passages in the valve and it quiets down. You run the risk of ruining the spider gasket however. In any case the noise is just annoying.
Al
Bookmarks