I have 2 "rattles" like this.
1 is the check valve from my Rainbow feeder, probably not the issue you have
1 is the multiport on the filter (same filter as you). I believe its because of air trapped in the system when the pump takes a bit to prime, as the noise only shows up when my pump basket isnt completely full with water (which happens either on first startup, or if there is a lot of junk in the skimmer or pump basket). Once the pump has run for a while the noise goes away. Ive also had some success running it on recirculate for a little while first, then switch to filter. Adding some back pressure would help if its that much of a bother.
Ive noticed that after you pump to waste when you push down on the handle it seems to let some air out of the system thru the glass viewing bulb, so Ive held the handle down and removed the bulb while listening for the air to subside. Not sure if that helps anything but it might.