Toil and trouble, from where commeth that insidious rumble?? I know!

[a77boy]

Allrighty, after reading this post you may discover that I am rather elated. If this has been previously posted before please forgive me, but I feel as though I have made a huge discovery. The rumble!!!! That annoying sound that 4Runner and Tundra owners have spoken of, that mysterious sound that seems to happen only at stoplights or when idling in gear. That irritating sound that somehow always goes away when you put the vehicle in neutral and take the load off the engine, yes that pesky rumbling sound.
After several attempts of trying to find the problem, including removing muffler heat shields and not limited to kicking around my exhaust system I think I may have discovered the culprit. Today while changing out my radiator fluid I had to take off the two separate skid plates to access the engine block drain valves. Upon removing the rear skid plate I broke two of the four bolts that hold the plate up, I was rather pissed but yet at the same time took that attitude of "eh, it happens" Nonetheless I was rather confused as to what to do. I could try and drill the bolts out myself but I figured they must of been stuck in there pretty dam good if two of them broke. I figured I would just run over to my local garage and have someone drill them out for me. I backed my 4 runner off of the ramps and headed out, with NO skid plates. Well what do you suppose happened??? No F'n rumble!! Is what happened!!!,, None to be heard!!! After stopping in to the garage to set up my appointment to have the screws drilled out in the morning I went on my way to investigate further.
I drove around for a good while to heat up everything to proper operating temp,, I tried stop lights, stop signs, and even the Wendy’s drive through (the rumbling always was noticeable there when bouncing off the drive through wall) and nothing!!! I never got it to rumble, Drove all over town and nothing!!! Ok, so I get home, flush and fill my fluid with good ol' factory red, reinstall the front skid plate and guess what I hear,, yes, the good old rumble was back!!! Go figure,, It was there sure as the sky is red! Anywho that’s my story and I am sticking to it. Now I just need to figure out a way to stop it. Knowing what it is certainly helps. I often banged that skid plate with my fist and heard a similar resonating sound, now it seems to be confirmed! Thanks for listening to my rant ;-)

[RobXS]

interesting so the skid plate acts like a drum?

[a77boy]

Yes, in a round about way it does. If you go down there and bang it with your fist in certain areas you can hear the resonating sound similar to what you hear when driving. It is a Pain to take the plate off, but on a lazy weekend take off both of them and drive around. Let me know your results.

[MrTom]

Interesting. Suspected it was due to some sort of harmonics being generated. Will have to attempt to get some of the rubber based stick on audio sound insulation (Dynamat) and see if it cures the problem.....

[a77boy]

Thats an interesting solution,, I thought about using some Teflon based washers to help dampen the sound where the plate screws to the frame.

[a/v1]

If the Teflon washers don't isolate it enough, and you want to go the dynamat route, you may want to forgo the sticky stuff, and go with a spray on dampener. Most brands have a spray on variant. Clean the skidplate very well. It's VERY messy, so watch the overspray.
I just don't think the stick on would stay with the truck. You could even use some spray-on bedliner for it.

[a77boy]

I put the skid plate back on and the nose is still rather annoying,, Thanks for the tip on the spray on dampner