without going to the dealer. Since my high-class infiniti has such a thorough trunk liner, I cant get to it from inside the trunk.

If I get in the back seat and tap the deck..it rattles...same sound i'm hearing going over coarse roadways and some bumps.

I'm not ever upset b/c the car is so quiet otherwise and I think it will be pretty easy to fix.

Any ideas on how to do it?

 

I took out the rear seats, and the rear cover above the 10 inch woofer in the rear deck and dynamatted on the top of the rear deck directly on the metal, and behind the rear seats on the metal and it's rock solid as well as no rattles whatsoever. Hope this helps.

 

I have the same rattle, it is intermittent and because of that is very hard to locate where it comes from exactly. It sounds like a piece of plastic hardware may be just loose enough to rattle when the right harmonic vibration comes along. I am thinking it might be a wire clip for the rear woofer. I have checked all over the rear seats, deck lid and the center arm rest looking for the source. I don't want to take it to a dealer just so they can tell me they never heard it. Frustrating to be sure, especially when the rest of the car is so tight and quiet.

 

I think you might be overthinking it. If you put a weight on top of the deck..it probably would stop the rattle. I basically want to put some foam under the deck that would put some upwards pressure on the deck. That would probably stop it...but I don't want to take out the back seats yet of my new car.

 

Follow up:
More investigation lead me to find that it is the rear passenger side corner of the grille for the subwoofer making contact with something underneath.
I have crawled into the trunk and pulled down the liner to check the bottom of the rear deck. The liner is anchored with velcro towards the front of the car. I plan on doing this one more time to see if there is an adequate size hole someplace near the rattle to force a block of foam in to stabilize things. I will post again on the results.

 

Well, here's the follow up. After fussing around with the rear deck (09 sedan) for several hours trying to locate the exact source of the rattle by tapping on it, I decided to go ahead and pull the interior panel which meant pulling the rear seat.
The seat bottom is real easy, pull 2 clips on the bottom front edge and lift. It comes right out. The rear backs are more work, and comes out in 3 sections, the left, middle arm rest and right side. There you need a socket for the nuts and bolts which anchor things at the bottom.
The plastic pop in clips retain the armrest and the panel and those are removed by popping up the middle with a small screwdriver, then the whole clip comes out.

Unfortunately, complete removal of the rear deck panel is more complicated since it has retaining ears from the lower headliner trim holding things in place. I did not try to remove it completely but lifted it enough to do the work, it requires that you pop up the 2 centered plastic anchors on either side of the speaker.
After numerous hours of trying to identify the exact source, I finally resolved to just add insulation and foam where I could without interfering with attachment or the speaker. I never pinpointed the exact source, but I figured the dealer probably would not have any better luck than I would.
I did eventually narrow it down some, and there were 3 possibilities. 1 - the seat belt retractor mechanisms ( but a rattle would still be heard when tapped and the seat belts were locked with tension on them). 2 - the plastic retractor covers (they might have been contributing). 3 - the electronics mounted on the rear deck ( I believe that the passenger side unit maybe a part of the vehicle dynamic control sensor which picks up motion, but I am not sure)
There is a LOT back there.
Eventually I had removed both the back seat and the upper trunk liner ( just more plastic clips and velcro). I took heavy closed cell foam and wedged it in between the retractor covers and the upper deck cross member to keep the covers tight into place (inside the trunk).
From the interior, I put insulation mat over the open areas over the top of the electronics to an extra layer. There is already some attached to the panel itself. Then reassembled everything and test drove it over the same PITIFUL streets that typically cause the rattle. No more rattle.
Admittedly I was not looking forward to doing any of this, but I am happy with the results. I have enough background in auto mechanics to be confident in my work, or I would have tried to take it to the dealer. To me risking the time going back and forth to the dealer trying to get it resolved just was not worth it. Rattles are notoriously hard to get fixed, they never seem to show up when you want them to and can be rediculously hard to pinpoint.
I must admit that I really did not give the dealer a chance to prove themselves, but oh well.

If you have a rattle in the rear deck, I would suggest that you try installing semi hard foam to wedge the retractor covers. (they do move up and down slightly and are not overly secured into place) That would be a fairly easy step, and doesn't require removal of anything other than the upper trunk liner. Plus you are not likely to gain any more rattles because you took stuff apart.

If that doesn't fix it, then check the child seat anchor covers to make sure that they snap down in the closed position. If they don't, you can always add a piece of tape to the lip to tighten up where it wedges into place.

With all the stuff that is mounted on the sedans rear deck, it really is not a big suprise that something should start rattling on coarse road surfaces, and after seeing what is there, Infinity does do a fairly decent job of insulating and securing stuff. But maybe they have better roads in JAPAN.

Nothing is going to hold up to the roads I drive for very long and be noise free.

 

I have that same rattle, mostly when I push the new 12 inch sealed / 500 watt RMS sub I just put in the trunk. But sometimes hitting a pothole causes the rattle, too.

I never thought to look at the seatbelt retractors, I will definitely damp them in some fashion.

I think the rattle originates -in my case- mostly with the trunk lining panels vibrating against each other / against metal body/frame pieces. I think using some closed cell foam tape here and there between the edges of the overlapping liner walls will be helpful- and also putting wedges, sheets, or blocks of medium density closed cell foam into the cavities of these lining panels to both damp them and keep in a certain amount of tension in their mounts - if a lining panel is under a bit of tension instead of just loosely hanging on it's mushroom pin mounts, it's constrained from rattling.

My trunk lid doesn't rattle until I push the subwoofer to a volume level of "11" and I never listen that loud so I don't think I need to fiddle with the trunk lid itself. It's all those liner panels that seem to be vibrating, and maybe also some of the back shelf bits.

 

I just did the same thing in my car trying to track down rattles going around corners. It turned out to be coming from under my driver's seat (see my thread in this section), but I basically stripped the interior completely to track down the source. Did you get the liner under the rear window out ok? You have to take off the C-pillar panels that cover up the charge canisters for the head curtain airbags. Aside from those covers, there are really only two white plastic clips on either side of the subwoofer that hold the entire panel in place.

When you put your armrest back were there just two plastic push pins on either side that held it in place or where there plastic pop rivets that attached it as well? I only ask because I have 4 plastic pop rivets left and I cannot figure out where they go. They look exactly like the ones holding in most of the felt trunk liner, but all of those are back in place. Hmm....