Remove the door panels:
and use Dynamat which has an adhesive backing: https://www.crutchfield.com/p_154104...-Door-Kit.html

 

i got all materials off Amazon after reading extensive great reviews. Dynamat was the original but there are equally as good and cheaper brands out there.

i did the full install myself taking my time to also sound deaden all the plastic tabs as well as remove the speaker grills and wrap them in tesa tape.

a lot of the resonance and vibration is coming from loose tabs heard at certain frequencies.

i had a full audio system installed and could of had them do it but figured i would do a better job and not go cheap on how much i used.

beside significant better audio i reduce road noise and interior cabin noise dramatically.

i prob went over board and i added two layers in the door panel areas with the first link and put additional foam layer on top.

only negative was a lot of weight added to the car.

 

Check this video for D90 with sound deadening vs stock:


My 90 certainly suffers from many of these rattles, but I would have to get a professional to address this level of work.

 

i only did the doors and there is plenty of videos on YT showing you how to remove. of course getting all the walls deaden is ideal but j did leave that to my installer as he had pulled all that out installing the rest of the system.

its not rocket science. like putting stickers everywhere. the amazon link i provided is very easy to work with. didn’t even need a heat gun. stuff is pretty sticky

 

How did yours turn out?
From what I can tell my rattles are coming from the rear panels, but I have not spent any time trying to isolate the source. My 90 has the Meridian sound system in it, If that makes a difference. Not adverse to doing a DIY, but don't want to put that effort in if the results are minimal.

 

i have a 110 but it turned out great and have no regrets doing it.

but there is sound deadening and addressing rattles. adding sound deadening to the interior door panels won’t address your rattles. rattles are caused by an object running or hitting something else or something loose. once you take the doors apart you will soon realize how poor the build quality is. some simple items of parts just not fully pushed in causing the issues.

i took around 2 hours per door but did the project over a couple of days

 

In my case my rattles are not general driving rattles. They only occur when the stereo is on at a decent clip which is why I'm looking at the deadening stuff. I guess it would be better term'd as vibration occurring due to bass frequencies. Regardless, I'm going to take a look to see how you pull the rear panels to try to put in some of this stuff. I won't get anywhere near to the level of effort these guys put in, but maybe it will be just enough to not annoy me.

First step though is for me to figure out where exactly the rattle/vibration is originating