Let me start by saying SORRY I’m sure this has probably been covered but the search function here isn’t the best.
My 2018 TRD Off Road with 50,000+ miles has developed a noise (sounds like 2 pieces of styrofoam rubbing together or possibly a dry suspension grommet) most of the time. I’ve had it in the dealership twice now to no avail. They suggested the spare tire might be loose, rubbing against the body. They also think it might be something I’ve added somewhere. Of course they can’t hear the “rattle”. I’ve checked the spare and it’s tight. I removed everything from the rear including the tool kit. No difference. The only time I don’t hear it is when it’s raining or damp.
This is my 3rd 4Runner ant about my 15th Toyota and my first with a rattle. I’m 78 and don’t get underneath to check things out like I used to. Anyone else with a noise like I’m describing…any ideas would be greatly appreciated. Mods, if you need to move this to an existing thread, please do.
Thanks Jim

Sounds like a bushing. Control arm, body mount, sway bar.

Buy cheap Frs radios. Use zip ties to have one always transmit. Then attach to different parts of the vehicle undercarriage. Should be able to narrow down the noise area.

I think first you need to identify if this is coming from inside or outside of the vehicle. there is Styrofoam in the trunk area under the floor (I think). does it sound creaky? perhaps it's the rear suspension and when it gets wet it no longer creaks. does the type of terrain make a difference

Also look at the heat shields on the exhaust system. Common source of noise since the mounting holes can rust out.

You mentioned removing the tool kit. Did that include removing the jack?

Did any of the tools fall out of the tool roll? One of those could be rattling down in the compartment.

Are you sure that the rear seats are firmly latched?

Same for checking the rear hatch latch although that one will trigger a warning light.

Check to make sure both rear coil springs are fully seated.

Check the bushings and bolts on all of the rear axle components - Panhard bar, upper and lower control links, anti sway bar bushings and shocks bolts and bushings. Maybe check the shocks too.

Get a couple of plastic car ramps and slowly back the 4R up onto them. This will make it easier to crawl underneath. Bring a flashlight and wear gloves so you can grab things and yank on them.

You mentioned that the sound goes away in wet weather. Methodically spray lubricant into one bushing at a time until the sound goes away then replace that bushing.

Possibly a heat shield. I had the same issue but it was the Jack kit from inside moving around rattling

I have a periodic rattle similar to your description. Had a friend ride along with me (back seat/ cargo area) to pinpoint where the noise was coming from. In my case the noise is coming from the headliner area, passenger side, rear seat grab handle. There are several likely culprits in this area: a thin wiring harness that runs atop the headliner (secured with tape), side curtain airbag assembly, and the grab handle itself. I took off and reseated the grab handle, which seems to have reduced the noise.