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Discussion Starter #1
I was sitting in line waiting at the carwash yesterday when I noticed this odd phenomena. The truck was in park and was idling when I felt this "thud". It was almost like someone tapped me from behind except nobody was there, so I shrugged it off. Well, as I waited in line it continued to happen, but it was not at regular intervals. The sterring wheel was straight and not turned either direction. It almost seemed that I could feel it more through the seat than anywhere else. Anyone else notice anything like this? Thanks for any help.
 

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not to doubt what you are saying, but are you sure you were in park and not idling in drive with foot on the brake? There was an issue about the driveshaft or something a while back when leaving a light or stopping..you would get a nudge and it felt like someone bumped you when actually its a part that needs greasing..I'll have to search around for it. I think it was the driveshaft/rear end. I get it in mine some times.
 

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Discussion Starter #3
Thanks for the reply. Yep, definitely in park. That's something I actually double checked.....
 

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I have felt the same thing with mine recently, thinking it's the slip yoke.
 

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did you have the defroster on? sometimes when mine kicks on I feel/hear a thump sensation
 

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ok, since in park this may be the A/C kicking on and off or something...as someone said above.. I've heard that before.

the slip yoke usually happens when leaving a stop
 

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Discussion Starter #7
Thanks for all of the help thus far. I'm in Wisconsin and it was about 35 here yesterday so the A/C was not on. I have been idling in park before and not noticed this. I just picked up on it yesterday.
 

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I've felt that too. Usually it's when I have braked fairly hard to a stop. At a complete stop with brake still applied, I have felt a little "tap from behind." But, nobody's been back there. I think it's just the driveline releasing tension or something.
 

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Same issue here. What is that noise? I have had it happen in park and when coming to a stop as well as when just leaving a stop. Some one must know...

Jimmy
 

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Still think it's the slip yoke. nose down, bed high during hard braking. sitting with brake on and feeling a thud. Could be the yoke slipping forward on the splines, while the rear settles back down, or the rear spring unwinding.

Just a guess though.. have not heard/felt it any other times.
 

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Been there, had that happen. No clue.
 

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Discussion Starter #12
Thanks for the replies. Again, this was in PARK with no brakes on.
 

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a/c will still cycle when on defrost had people with concerns of
that a/c kicks on hear thud/thump the cycles off other than that
this is a hard one to think about because your doing absolutly
nothing.maybe heat related body/bed popping from heat not sure?
good luck
 

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I noticed mine doing this the other day when returning from a long trip pulling the 5er.. I was sitting still in a parking lot. and it happened 5 times durring the time I was parked.. If it had been the drive line it would only have done it once or at the most twice...

I did not think it was coming from the AC unit though.,..
 

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Most likely its your locking diff ingageing,your diff is unlocked at speeds above 25 mph and when you stop or make a turn below 25 mph and straighten the truck up and come to a stop the clutch packs engage (hard) and you feel the thud your refering to.All my trucks I have owned and including my wifes tahoe with locking diff does it.
 

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I'm having the same problems with my truck. It started 3 weeks ago and was occasional, now it's all the time. It happens whether I'm in drive or park, whether the engine is cold or at operating temp, whether I'm moving or stopped. My wife first noticed it as a "thud" in the floorboard under her feet on the passenger side. I found another topic here that suggested a bad injector. I'm going to try to scan my engine first of the week (after the snow melts), and see if anything shows up. I'm convinced it isn't the transmission or suspension, and since it occurs with the truck in "park", I seriously doubt the driveshaft or rear axle.
Keep the ideas flowing...I need help. I have to drive 35 minutes to the one dealership I trust with my Sierra, and they always want me to leave it for a day or two, which turns into a week. I don't have time for the stuff, so I need info to get them motivated to move fast.

Thanks in advance,
 

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Most likely its your locking diff ingageing,your diff is unlocked at speeds above 25 mph and when you stop or make a turn below 25 mph and straighten the truck up and come to a stop the clutch packs engage (hard) and you feel the thud your refering to.All my trucks I have owned and including my wifes tahoe with locking diff does it.
Sounds good, but up until 19K miles on my truck, I never experienced this. Now it's all the time...a dramatic thud in the floor board over the right frame rail. I've heard the clunk you're talking about...at least in my case, what I think we're discussing is something entirely different. Why would the diff engage/disengage in "park" at idle? I will say the knocking I'm experiencing is consistently below 25 mph. Why start now? Why not when new? I don't think this is something I can comfortably "accept" as normal for this truck.

Thanks for everyone's input,
 

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Mine just did this 2 days ago on my way home from work. It felt like it backfired or something! under the passanger floor is where it felt like it came from. sounded like a solid thud! I was going 55-60 scared the hell out of me I honestly thought I ran something over.
 

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It is the transmission... The Allison's do a sort of self test while in neutral and in Park. It tests the C5 apply many times to "learn"...

It actually applies a gear while in park, and you can feel it. You can watch the turbine speed drop to zero when this happens with a TECH II. It's just testing it's controls.. Pretty neat :)

Transgo shift kit'd guys will feel it much more than the regular guys.
 

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In response to above.. The eaton G80 "Gov-Lock" type locker does not contain any clutch packs. It's a gear type locker.
 
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