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This is my first time on this site so I am sorry if this post goes somewhere where it shouldn't, I am still learning my way around the site. I just purchased an 05 Envoy and with our first snowfall in the Northeast I put the Envoy in A4WD. There is a terrible thumping noise when using this that is not heard when driving in 2WD. At first I thought I had a flat tire so i got out and checked-No flat tire. I haven't tried to see if it does it in 4Hi. It is really loud and annoying! Any ideas as what this could be. I have not had the vehicle a month yet and I am not happy that I am having problems already..there is 50,000 mi on the vehicle.:hissy:
 

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Discussion Starter #3
A4wd

It does also make the noise when you first put it in A4WD but it continues as you are driving down the road. It is not constant but it is frequent. When I leave work I will try it in 4Hi and report back to let you know if it does it in this as well. Thanks for the reply:)
 

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2003 chevy trailblazer_ls
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Is it doing this going straight or only when turning? Is the road covered in snow or is it patches of snow and asphalt? 4hi 4lo drive does not like asphalt and should never be used on asphalt. However A4WD should be ok either way. Roadie has a drawing on why and I need to save it for future use. You are due for a transfer case fluid change and that might help.
 

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2004 gmc
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I agree, inspect the level and definitely change the TC fluid. Low or old fluid can result in grabby clutches, and that's what I think the thumping is. Read the Owner's Manual if you're new to the system, and if you didn't get one with the vehicle we have many under the "Resources" pull-down menu. It details the operation of the A4WD system and contains more warnings about using 4HI and 4LO on surfaces with good traction.

Everyone reading this should be exercising their 4WD system every few eeeks, even if you're not using it. You can do it in neutral with the engine off, and you should listen to the various noises the system makes as it changes modes. Observe the flashing and steady lights on the mode switch as well. If anything breaks later, remembering how it worked when it was operating well is a big part of the troubleshooting process. And when you post about how it's broken, I always ask - well - how are the engagement noises different from before?
 

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2002 gmc envoy_slt
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Wow I've had that thumping noise on occasion but I would be in 2hi. I would start up and then about 15or 20 sec. later it would happen ,the first time it happened I thought some one hit the back of me.
 

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2002 chevy trailblazer_ltz
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I have an '02 with 190k that does this when the 4wd engages or disengages. I will be checking the fluid this weekend, but I keep up on it well so I am leaning to a universal joint in my case. Just a reminder to everyone to check the U-Joints while you're under your rigs.
 

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Wow I've had that thumping noise on occasion but I would be in 2hi. I would start up and then about 15or 20 sec. later it would happen ,the first time it happened I thought some one hit the back of me.
This has happened to me too. Usually when I stop at a traffic light (in 2WD) and 10-15 secs later it feels like someone hits my back bumper! The first time this happened i was sure of it but the car following was far behind me.

This sounds like a different problem, but similar to this thread post. Would this be a cause of the clutch grab problem roadie mentioned??? '06 has 40K miles on it. time to change the xfer case fluid??

cheers
 
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