Hm, ok so you've opened the grate in the test chamber, put the weight on the pedestal, kept the gun, then dropped it into the grate in the testing room.
*puzzle spoiler ahead*
Then you went to each floor, opened the correct grate (you don't have to open them all) and flipped the vent/bypass by using the pipe on the little flu looking thing in the vent (when you click on the grate, you see a close up view of the grate, which actually will have the gun lodged in the vent) so the item drops.
You did this on every floor down to Maintenance.
Then you fixed the power, and used the pipe on the hinge where the rattle is.
Were all those steps taken?