Thanks -- I believe I ruled out the vacuum relief valve. I don't see any other sources of air intake on the ground so it may be on the roof. Also -- when I bypass the panels -- I don't get any air -- so I don't think it's a pump / filter leak.

It may be lower pressure? I just had the filter replaced (same 1 HP pump) and the pressure is now lower - I originally thought that was good -- clean cartridges -- but not sure.