I just want to update you guys, so I went yesterday to the dealer where I got my Accord to complain about the rattling/vibrations on the rear deck subs. To my surprise, the service advisor said that they can’t fix it under warranty because I tinted my windows. I told him that the problem was already there even before I tinted my windows but still they said they can’t fix it under warranty. They could however fix it with a labor fee of $60/every 30 mins! I was like... **** No!
So today I tried fixing it myself just like UGotWorkedSON’s advise and countless of similar tips I found across different Make and Models with the similar problem. I used some egg crate-looking foam I found from some package shipped and cut 1/4” x 6”. I then squeeze this foams on every hole underneath the speaker deck which you can see when you open the trunk. Note that I didn’t disconnect or unscrew any part of the deck. I just tried my best reaching it with my fingers. After I filled the holes with foam I then tested it out and played some heavy bass hip hop songs. Voila, the vibrations have disappeared! I’m able now to crank my bass/volume up and adjust the sound settings to how I want to without worrying about the rattling! Thanks for the help guys!
I should mention, I had this same issue with my old 07 Civic, and fixed it the same way, took some pink egg crate foam I got from work, jammed it in the little oval holes on the underside of the rear deck and it fixed it mostly.
My other issue now, even without having music on, is the freaking trim around the HUD projector squeaking/rattling. Since I have a black dashboard, I'm thinking of taking some black craft foam and jamming it in between the trim and the windshield to press the trim down.
It's kinda BS that they make these cars so hollow now for weight reduction and fuel economy, but yet you can't even enjoy the ride because everything makes a noise.
Ok, end rant, but just food for thought, try using black craft foam if you need to jam something between the rear deck and the rear windshield if you have a black interior. 99 cents a sheet at Michaels or your local crafts store, it's about 1/8" thick.