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The weird clunk

T

TurboTahoe

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Hi guys,

Lately the Tahoe has been exhibiting some odd noises (and no, they aren't coming from the driver). When taking off from a dead stop, shortly after application of the throttle, you hear a slight 'thunk' and there is a little 'shift' felt in the drive train.

Putting the rig in Neutral causes the behavior to go away - that is, application of the throttle doesn't give any thunk or shift.

It is definitely not the 1-2 shift transition (at least I don't think so), but it feels like it is coming from the driveline. If I keep the transmission selector in "1", it still does it, so I assume it not a shift-thunk, as I am keeping it in 1st gear.

If I hold the brake pedal firm, in drive, and give it some throttle, then it appears to do the same thing - shift sensation and small thunk sound.

What could it be? Is it the engine mounts getting old? If so, how hard to replace? What else could it be?

I tried this with the brakes on and with hood open. I can see some shifting of the engine, and it appears that the 'thunk' is synchronized to the shift of the engine as it torques up.

Educated guesses and experience solicited. Thanks, guys.

-Rob :)
 
Matt Bachand said:
E-brake dragging and releasing?
Click to expand...

Wow Matt, good thinking. I don't think so. If this was the case, it would not clunk when I have the e-brake set, Hydraulic brakes on, and still clunking when I apply the throttle.

I'm starting to convince myself it's the motor mounts.

Has anyone changed these out? They are hideously cheap (in the $30 range), but look like a bear to replace.

-Rob :)
 
Could be U joints, check that they are easy; but you say it clunks with engine power, exact symptoms as mine 3 mos ago, Pull out the rubber skirt in the fender well, insert a crowbar or tire iron under the engine mount and give it a lift, see if the rubber is separated from the mount, if you have never changed them I'll bet they are due, common enough happenstance on the 6.5 when they get some years/miles on them, both of mine were gone, replace with rubber, not nylon/urethane, several guys went with hard mounts and regretted it, too much engine vibe transmitted
 
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Give em a shot! In fact might as well take pix and make it a write - up for the rest of us.

Going Delco? I heard the 'high performance urathene' mounts are too stiff and you feel the engine vibration throuhgout the cab.
 
Turbine Doc said:
Could be U joints, check that they are easy; but you say it clunks with engine power, exact symptoms as mine 3 mos ago, Pull out the rubber skirt in the fender well, insert a crowbar or tire iron under the engine mount and give it a lift, see if the rubber is separated from the mount, if you have never changed them I'll bet they are due, common enough happenstance on the 6.5 when they get some years/miles on them, both of mine were gone, replace with rubber, not nylon/urethane, several guys went with hard mounts and regretted it, too much engine vibe transmitted
Click to expand...

Thanks Tim,

As always, you have great suggestions. It's a good excuse for me to buy a big crowbar! (I know, I can't believe I don't own one...)

Matt, thanks for the encouragement. I'll report back when I do the crowbar test!

-Rob :)
 
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Actually the noise you are describing could be the trans mount. They are known to sound like that. Also check for your downpipe hitting somewhere. I see you have the over size one. Checking motor mounts is easy. Have someone power brake it and then observe movement in both forward and reverse(From a safe position). You can usually see them move if they are broken or badly worn.
 
Acesneights1 said:
Have someone power brake it and then observe movement in both forward and reverse(From a safe position). You can usually see them move if they are broken or badly worn.
Click to expand...

That works if you got somebody, here in Ga it's just me, actually way I 1st diagnosed it was started the truck put in park then jumped up & down on front bumper, and could make it change tonal pitch,I shut it down & then I cut wheel hard an used a prybar to see if the mount was shot, it/they were.
 
It could also be the tranny flexplate taking a dump. When the '99s broke it sounded horrible... If the clunk gets worse this would be my guess. Also it was only really noticeable taking off from a stop / low rpm idling.

... Mine was caused by the PO not reinstalling all the tranny bolts!!! Grrr.
 
My 99 Tahoe did just that pretty much every day of its life. I brought it in to have it "fixed" numerous times and within a few weeks, it always came back. Its supposedly called gear lash and its from lube drying out prematurely between the axle/u joints and rear end. There are splines inside there that get dry and the clunk is the complete lack of lube. Its plagued GM trucks for nearly 20 years that I know of. Every fleet GM truck, 90-99 has done it to include burbans and Tahoes. In fact, my 04 GMC DMAX did it and my 07 clunks too and has done it since 2500 miles. Its gotten worse over time and so did my Tahoe. I've had my 07 in the shop twice to get it fixed and it keeps coming back. In fact, I get it in reverse too and so did my 99. Its a poor design if you ask me.
 
My intermittent "thunk" turned out to be one of my fuel tank straps being loose.

My buddies 89 gasser had the same problem, -he's the one that gave me the hint, -and it turned out to be what it was.
 
Use a helper to run the truck and get the thing to "Thunk" while you snoop around and look things over.

Another thing that can cause a Thunk is the torsion bars up front.
As the power is applied the chassis flexes some and the bars can move slightly in there mountings and then pop loose making the noise.

This happens more when things are dry than wet as any water in the joints will tend to lube things a bit.

Motor mounts are always a suspected item as is the T case/tranny mount.

Rear spring bushing or u bolts that are loose allowing the axle to move on the center bolt.

Good luck and keep us posted.

MGW
 
clunk

If all else has failed to correct this clunk, try removing drive shaft and putting a small amount of grease on the slip yoke splines. I think that may have been what UNIT453 was talking about. Has been a thorn in GM,s side for many years. Was a grease released for this many years ago but no clue what part number.
 
Agree, since you can make it do the noise on demand, have someone else do the noise and crawl around under there, your hand can 'hear' really well.

Chock blocks and E-Brake's safe.
 
I've had a similar clunk that sounds a lot like yours. I've described it as feeling like a manual seat that is in between stops and slides back and locks into one on take off. I only get it when taking off after coming to a stop from speed. If the truck has been sitting and I take off, no clunk.

I've thought I needed to check out the rear spring packs for a cracked one, and eventually, the spring pack locating pins, as it "feels" like the rear end is shifting around under the truck when this happens.

Unit's post makes me think I should check the slip yoke - I know it's something to keep an eye on on the Dmax, just hadn't thought about it on the Tahoe. Plus the motor mounts and the tranny cross member....ah the fun never ends.

Good thread, some good ideas here, thanks.
 
I only suggested that because its happened exactly how he describes on every GM truck, since the early 90's that I've ever driven. Its a noticeable clunk when leaving a light, after a complete stop. I can feel it in the pedal. You would think after all these years that they would've fixed the problem.
 
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