I did manage to fix my passenger side door rattle (2018 i30n hatch with around 35k km on it). I've been driving maybe a few thousand kilometers since the fix and so far so good.
I understand this is quite a old post but I'm replying anyway in hopes that this helps someone in the future. This will also be a long one so skip to the end if you don't have the patience. I understand that I might sound nit picky but this sound
absolutely drove me insane. To the point of considering selling the car. So here goes.
These were the symptoms of the rattle. These happened on and off with no clear sign of conditions which triggered it. Although I noticed that they started to be more and more constant the longer it took for me to fix.
-very audible buzz when the bass hits (think like having an empty water bottle in the door side pocket and it rattling against the plastic)
-like someone else said, if you pressed on the door armrest, this basically eliminated it. (this is a bit contrary to the fix I found, more on this later)
-I also had similar sounding rattle when driving on rough bits of road, coming roughly from the same area. This was the biggest reason I was so annoyed about the issue
-I also noticed kind of springy, rattling sound when you hit the door speaker area with your fist when the doors open. Very different from the driver's / left side that is silent
So first thing I did was have the local hyundai mechanics take a look at it (around 30k km driven at the time). No bad word about them really, really good blokes all in all. But no fix. This might be partly due to me being very vague about the sound, not connecting the bass related part and rough road related part. They basically answered that the car's made of cheap plastic and it rattles. Some more and some less.
Okay so after some consideration I figured that maybe the two conditions that it occurred under were related (the bass and rough roads). This is also when I found this thread.
So I decided to give it a go, took the door apart and see what's inside. It's very easy to do, basically held on by one screw and a bunch of plastic clips that come apart with a bit of force. Here's a video (not mine) that explains it clearly:
The first try I didn't have much luck so
I don't recommend doing this as a first step. I had some sound deadening matt bought from general auto parts store. The one with basically glue on one side that you stick on. I basically covered the whole area between the door frame and the plastic panel with the stuff. So gluing it on top of the white OEM sound material.
This did not fully fix it. Everything was silent on quick test drive and blasting some music on the driveway but after just a few hundred kilometers it started again. So maybe it helped some, maybe it didn't. There was also some difficulty getting the door panel back on with all the extra padding so I had to remove some material here and there before getting it snuggly back on. I pretty much only included this step to be as thorough as possible. But if you do the next step and it still doesn't help, maybe this is something you could try next?
TL: DR
So finally to the solution:
I took the door panel off again.
(Here's the video that details how to do it, very simple)
When I had the plastic door panel assembly removed and and started poking around I noticed that there was some slight movement (+-1mm) on the electric control module that controls the power window switches (and I think mirror controls too).
This is most likely the problem. I did some additional steps while I had the door apart but I think this was the most likely problem.
I didn't figure out how to get the box removed from the door panel completely. I think it's held on to by more clips on the other side of the module and those are near impossible to reach. If you figure out how to get the box off completely there might be some "cleaner" way to pad it but what I proceeded to do worked just fine for me.
I just so happened to have some race bicycle inner tyre around from the "how to fix your windscreen rattle" and so I sliced some narrow pieces of that rubber and just basically shoved those between the clips. Be careful as not to snap the clips, I used a very thin, dull blade to work the rubber between the clips. If you don't have any bicycle inner tyre around, look for the thinnest one you can find from your general auto parts store. There's probably some better material to do it but this is what I had on hand and if you buy it you can also do the windscreen rattle fix.
While doing this, I poked around some more but I don't think the next parts are as vital as the first. But since you have everything apart, check these as well.
Additional steps:
I noticed some looseness on the clips that hold the entire panel on. This might be due to it being taken apart several times already (once before by me and at least once by the previous owner I think). I also suspect that some of the clips weren't original (there was different colour clips but some were plain white, I don't know if the white ones are OEM?).
I had some rubber seals from clothes washing machine inlet tube and those were pretty perfect fit so I put them beneath the most loose clips. Basically pop the clips off carefully and put the seals between the clips and the panel. I'm not sure if this matters since after you pop the panel back on, the tension on the panel might keep these from rattling but just in case.
I also noticed that one of the wires that clip to the door side (not the plastic panel) was pretty loose on it's clip. This is the one that does not seem to lead anywhere, it just ends to the bottom right corner of the door. I dismantled the whole clip and padded it with some sound deadening tape. Again, I don't think this was the main culprit but just in case.
And as the last precaution step, I sprayed some silicone spray one all the other clips that connect to the door that keep the wires in place. There was not much wiggle on those but just in case.
Makes sure to not spray the connection points, cover them with paper towel and be careful. Also keep some paper towel on hand if it drips.
That's it, hope this helps someone! I also have some pictures on my phone so ask if you need those or any help at all.
An afterword:
After couple thousand kilometers, no rattle from the door area, absolutely none. I've noticed a few more minor rattles though coming from the rear of the car and also something in my glove box... Oh well, you've got to have something to work on. I also wish that I'd done this whole thing in a more controlled matter, step by step, just to write a better guide. I'm just relieved that I got it all worked out though.
