knocking from under cargo area

[Dan of Steel]

2005 sport edition 73k miles,
I have an intermitent knocking that happens randomly. At first it sounds sort of like a flat tire. It will start gradually then gets pretty loud. It seems to speed up and slow down with the speed of the car. the speed of the knocking seems slower than the RPM of the wheels. One time when it was doing it, as I was slowing down and had almost come to a stop it started popping really fast like a machine gun. Sounds like something hitting the underside of the body in the cargo area. It still does it if I shift to neutral and coast. I drove about 1 1/2 hours yesterday no problem then it started. I stopped a couple times turned off the engine. Then finally, it just quit doing it. I then drove 1/2 hour back home and no sounds. I don't know how to make it act up. everytime I take someone for a ride to hear it , it won't do it.
I noticed a box mounted under the cargo area above the spare and wondered if it could be coming from there. Does anyone know what that is ?
I have a good audio recording of the noise but can't find a way to attach it on this forum. If anyone is interested in hearing it to help diagnose the problem, I would be happy to email it. Please help !

 

[20034RunnerV84x2]

Is it the spare? It could be loose.

 

[kevinpt]

is it the spare? It could be loose.

..

+1

 

[Doozi]

I agree... it's probably the spare. I came out of a local tire shop when I got my tires (all 5) and heard some noise... knock, knock, thump, etc. I pulled over to check the spare and found that it was loose. The carrier cable has a spring on the end of it so I always make sure the tire is centered and I load the spring somewhat to keep it under tension. That box mounted above the spare shouldn't be making any noise (unless it's falling off)... I believe it's all electronics.

 

[Dan of Steel]

No way. Is not loose spare. This is loud knock and speeds up / slows down as the car does.

 

[Dan of Steel]

I said it happens "randomly" but when it does happen , it is steady knock for a while until it finally stops. And the one time , as I came to a stop, sounded like machine gun ...... or pneumatic impact wrench removing lug nuts.

 

[riceandpho]

check the nuts on the shield above the muffler, and also check the muffler too.

 

[Radial GT]

Check you roof rack cross bars. Mine were loose and it would make crazy noises like that. Go up on the roof and hit them with a fist to see if you can recreate the noise. Noises in vehicles are hard to nail down. Just a thought!

Good luck

 

[TechWrench]

I would check the driveshaft, U-joints and flange mount to differential/trans.

 

[Dan of Steel]

They guys at the repair shop could not get it to do it and think it was the emergency brakes. They said the emergency brakes were not working so they adjusted them. They said the emergency brakes were also worn as if someone had driven with them on. They were not on when it made the noise. A buddy of mine that is a mechanic heard the recording and said no way brakes would make a noise like that.
If there is a knowledgeable mechanic reading this that is willing to listen, please let me know and I will post an email address so we can make contact and I can send you the audio clip.