help me with this rattle!

One of the rear vents in my 95 civic ex keeps rattling like crazy when i turn the bass up. Its basically impossible to slip dynamat pieces into the cracks (thats what i did for other places) around and under the vent bc its so close to the rear windshield you can't get your hand back there. Should this vent pop off if I pry it, like is it basically just clamped down?

if i can rember correctly from those there should be a couple 3 or 4 10mm bolts you can get to from underneath (in the trunk area) take those off and it should slide straight out...otherwise it's a pop