Odd vibration at low speeds

[TahoeInFC]

Hey Folks,

I'm back with yet another weird question. As long as I own this truck, I'm guessing I'll be back again and again.

Lately, when my 99 2dr Tahoe is cold, I hear a faint vibration, and can feel it beneath my right foot. It sounds and feels like something vibrating right behind the firewall. Doesn't do it as the engine and transmission are working up through the gears, but I feel it when it drops in to 4th gear and the idle is sort of low. That's when I really feel it. Goose it a little or let off completely, and it seems to go away, at least partially. It seems to be tied to engine RPM's, or some other moving part. It does this at city speeds (40-45mph). I don't know if it does it at highway speeds...haven't heard it anyhow.

When the vehicle warms up, or on a warm day, nothing. It runs and drives very smoothly with no odd vibrations beneath the feet whatsoever. This leads me to believe that as things heat up, whatever IT is, it expands with temperature and can't move as much, so stops 'buzzing'.
Although, the troubling thing is that when it's cold, it seems to be getting more pronounced.

This truck is still stock, except for a new muffler (like a Flowmaster 50 series, or similar) and larger pipe to the rear. Not stupid loud, but definitely adds vibration to the system. Still a single outlet. The 'Y' and cats are the same as before. The muffler change happened several months ago, but this noise is new.

So, I did some hunting out here and found some similar complaints, but no resolutions. Maybe I missed some? Maybe you guys are collectively shaking your heads and thinking 'poor bastard'. I don't know. Either way, I'm relying on your expertise here.

Some thoughts that crossed my mind as I was reading other posts:

1) Different exhaust system vibrates things, and something has started to come loose. When the exhaust system heats up, it doesn't vibrate as much and gets quieter.

2) A mount somewhere is loosening gradually, and stops vibrating for the same thermal reasons.

3) A U-Joint is starting to fail at colder temperatures.

4) Transmission/Transfer case issues? I pulled the tranny dipstick and fluid is up to level and very clean...not burnt. I don't think this truck has had the transfer case pump rub fix installed that I know of. It's on my list. Could that be going south on me?

5) Other horrifying and expensive problems?

Again, this thing runs and drives great. Seems to have plenty of power. Would just like to figure out what this is and head it off before something falls off!

Thanks in advance!!

 

[SunlitComet]

are we talking like a thump every complete rotation of the crankshaft kind of vibration?

 

[drakon543]

Also what kind of cold are we talking? Does it stop if you leave the truck idle and warm completely or warm up as your driving. If you live in an area thats still dipping into the 30s at night it could just be your tires. When its cold and it sits over night sometimes the tires can get stiff and have a flat spot for a mile or so. Once the tires warm up it goes away. Also when i was running a flowmaster on my suburban it did that for a while. I found one of my clamps had loosened up a bit when it was cold the exhaust would vibrate.

 

[Kpwweb]

Also what kind of cold are we talking? Does it stop if you leave the truck idle and warm completely or warm up as your driving. If you live in an area thats still dipping into the 30s at night it could just be your tires. When its cold and it sits over night sometimes the tires can get stiff and have a flat spot for a mile or so. Once the tires warm up it goes away. Also when i was running a flowmaster on my suburban it did that for a while. I found one of my clamps had loosened up a bit when it was cold the exhaust would vibrate.

What they said ^^^

Tires flat spotting or exhaust. Might be cracked exhaust manifold, but probably doubtful?

 

[TahoeInFC]

Hey folks, thanks for the replies. Sorry I've been slow to respond...

SunlitComet, no it doesn't really feel like an engine thump, but I suppose it could be since it seems to be correlated to revolutions. I've had other vehicles with nasty engine vibrations (bad motor mounts, running with a miss or two), and this one feels a little different. More of a buzzing than a thumping. What're you thinking?

Drakon543, the exhaust vibration you talked about seems fairly likely, although I haven't crawled around under the truck yet to see if I can find something loosened up by the muffler. The truck sits in the garage all night where the tires are kept between 40's and 50's when it's in the 30's outside. I'm guessing it's not the flats from cold tires, although I have been surprised by these things before. I had an old car with bias ply tires that sat outside in the winter, and when I drove that car, for the first few miles it felt like I was driving on square blocks instead of round tires.

This is going to be a fun one to catch in the act. This morning I got in and drove, and it didn't do it at all. Smooth as silk. Next time it might. Hmmm...

 

[TahoeInFC]

Hi All,
Well, finally had the opportunity to crawl under the Tahoe to see if I could find anything loose.
Indeed, right under the gas pedal on the engine side of the firewall, there is a heat shield. It sits between the firewall and the driver's side exhaust pipe leaving the engine. This heat shield makes a little rattle when thumped, so I think I have my culprit.
Now, how to solve the issue. There are a couple of screws holding this heat shield to the firewall, screwed through a seam in the body metal. These screws seem to be tight. Not sure why the shield rattles.
I'm wondering if I go to the local parts store and get some of the high-temp silver thermal shield 'blanket stuff' and push a sheet of it between the heat shield and the firewall, if I can stop the vibration. I might need some high-temp adhesive of some sort to get it to stay in there.

Any of you clever folks have other ideas?

I think this heat shield is picking up exhaust pipe vibrations and finding its resonant 'happy place' under certain engine loading conditions. Usually light load at low RPM's, like after shifting into 4th gear at 40mph in town.

 

[drakon543]

Well if you want to try that heat shield idea just stuff some paper towels or something in there for a short trip around the block. If it fixes your issue then go with that thermal packing. Im not suggesting you try to drive around town with paper towels stuffed next to a heat source but just long enough to see if it would solve the problem. Throwing money at various locations of the truck till you get your problem without a firm diagnosis can be costly.

 

[drakon543]

I do not however think that is your issue but without finding anything else you might as well try it.

 

[TahoeInFC]

I do not however think that is your issue but without finding anything else you might as well try it.

Interesting idea with the paper towels. Maybe I could use steel wool or something like that just for something to try and can handle heat.
I'm more interested to hear your theories on what it is if not the heat shield.

 

[1998Suburban]

I had a nasty resonance at 55mph that took forever to find. It was a bad u joint in the front d/s. The front d/s spins even when the 246 transfer case is in 2WD. It isn't transmitting any power but can still set up a vibration.