Just take the cover off. The rattle when the cover is on is a well known problem with the aquaclears. It's a hit or miss with the rattle. I've owned quite a few ac's throughout the years and I would say half of them rattled.
After farther investigation it appears the inlet tube is to long causing it to touch the lid. I think filling down the tude a hair might be the fix. More to come..
I don't think you have the tube pushed down all the way. It shouldn't be touching the lid. Make sure it's snapped in all the way and try pushing it down while rotating it back and forth.
My lids do rattle from time to time. Usually just touching them to resettle the lid will cause mine to stop for awhile. If that doesn't work I just take it off until I happen to notice it again, then put it back on.