Holy cow... I've read several articles here about a humming sound or a harmonic sound, but this wasn't any of those (That I saw) and it was INSANE.

Been driving for 3 days in the mountains, little offroading, tons of cornering for hours. On the way home, on a super twisty turny road when a gawd awful droning harmonic sound starts up. Like a 1kz tone, from the rear end. Speed made it louder but no frequency change at all. At 40 MPH it was DEFENING.

We were hours away from any kind of mechanic or dealership didn't know what to do. Didn't feel anything, no vibrations in the stearing sheel, brakes or gas. The sound would stop when I stopped the car. Looked around underneath several times, nothing loose, nothing leaking, nothing rattling. Car is only 3 months old.

Every time I got back in the car, the sound would start again at about 20 mph and got deafening at 40. It wasn't related to anything in the drive train itself and frequency never changed, just the volume with speed of the car (not the rpm of the engine at all).

When we got to the highway I sped up and from 40 to 70 the sound didn't change but > 70 it faded away. Thankfully as we were 5 hours of freeway driving from home and we would have been mental by then having to listen to it for that long.

Well after a couple hours on the freeway we stopped for some food and gas. By then I was convinced it was the exhaust, like the baffle tube broke off when offroading and was causing the noise. While pumping gas I started gently kicking the tail pipe and the muffler to listen for clunks or loose movement. I then kicked the joint where the two post cat tubes meet before the muffler, nothing, everything seemed really solid no noise at all

Except my wife said that inside the car she heard a small "tink" sound when I kicked at the joint, like a pebble hitting the underside of the car or something. I tried kicking at it again but no more "tink"s. We got back on the highway and as suddenly as it started, the noise was completely gone...

Double-you Tee Eff?

Edit:

A short video of the sound as we were driving along, and this was only about 20-25mph or so, at 40 it was far louder.

http://tinyurl.com/mfj3a8l

I had a similar thing happen recently. Freeway at 55-65 mph. Drop under 55 went away. Was a very windy and rainy day. Been on freeway since and it didn't repeat. But it wasn't rainy or windy. Thought it might be related to my new baja rack. However The noise reminded me of extreme tire hum. Also reminded me of the noise from a work truck when the the two way radio antenna iced up on top of the truck. Hum was unbearable at highway speed until I broke the ice off. So for now no clue what caused it. Weird that someone else had a similar experience in same time frame.

I just edited the original post with a short video with the sound. Did your's sound like that? For us though it was all perfect weather :/

You wouldn't happen to get the vehicle undersides muddy while off roading did you?

No mud, it's dry as a bone out here. Tons and tons of dust though And possibly a few small rocks thrown up and around under there...

No mud on mine either. Safari won't open your link so can't listen.

It's a wmv file. Didn't have an easy way of stripping the audio and making an mp3 :/

i'll have to sneak onto the site at break tomorrow at work and use a different computer to access the link. My mac and ipad don't have software to run this.

Ever been out riding a mountain bike and had trail debris wedge up somewhere between the wheel and chain-stay or fork. Even a leaf can make a big racket.

I would guess something along those lines, but most likely a rock or stick since it was clearly speed related.

That IS loud and irritating! That would drive me bonkers if I had to listen to it for 5 hours. Could something wedged on some sheet metal?

I had tie straps on my roof tying down some stuff of the roof rack...the straps fluttered when I got up to 20+mph. The fluttering straps echoed throughout the interior like a guitar string playing a note on a guitar. Were you running tie down straps on your roof?

Anything on the roof rack?

I had a random thump/thump drumming sound that I couldn't figure out. I swore it came from the back of the truck.
Nope- it was the rubber strip under the GOBI wind fairing. The strip split at the seam and was sometimes sitting on the roof, sometimes flapping around up there.
The origin of a noise is hard to pinpoint!

I concur with these opinions. When my mtn bike picks up a leaf it sounds like someone put a playing card in the spokes! I have had similar noises in the past, all caused by some random twig or something vibrating against the floor pan or driveline. We all could be wrong and your 4Runner is about to blow but I doubt it lol.

Passed over some debris recently on a forest-access road. Minutes later I'm hearing a light scraping from the rear. A small limb had got stuck underneath and was barely dragging/scraping the road. Drove me crazy.

Download Flip4Mac and it will work...

Probably nothing in the tire or front axles, as that would have changed pitch with speed.

Nothing on my rack though I HAVE heard that sound before When I have my high-lift on the rack I use a rubber strap to keep the foot from rattling and when there's a crosswind that thing hums surprisingly loudly.

It seems like it must have been a wind caused resonant humming like straps on a rack, but from under the car somewhere, though I looked under there at least 3 times while it was making noise and there was no forein objects visible.

I'll take a good long look today after work but It may remain a mystery for the ages