Strange jolt / thumping sound at start up.

[DeepSeaFish]

My 06 Denali XL does the strangest thing at times on a cold start. Seems like every 15-20 cold starts or so, I'll get that one start up where something jolts the truck a little making a loud thumping sound right as the engine fires. There are no codes stored in the system and everything else is working normally. Transmission shifts properly and the fluid is clean. Again, it's only maybe every 15-20 cold starts that this happens. Then it wont happen for another week or two and DOES NOT happen on warm engine starts. Any ideas? Judging by the sound and feel of it, I'm guessing possibly a jolt from the tranny... But I've never seen this before.

 

[06xl]

I have the same issue it usually happens like you said on cold starts but I've realized it happens when you put the key in the ignition and immediately turn the engine over what I do now is turn the key to acc and wait for 5-10 sec then turn engine over haven't herd then noise since

 

[DeepSeaFish]

Glad to know someone else has the same issue. I'll try the 5-10 second thing and see if it works. Cranked it up this evening after sitting for about 9 hrs...cranked fine. Dealer can't duplicate because it's not happening that frequently.

 

[06xl]

Glad to know someone else has the same issue. I'll try the 5-10 second thing and see if it works. Cranked it up this evening after sitting for about 9 hrs...cranked fine. Dealer can't duplicate because it's not happening that frequently.

That method has been working for me so I just continue with that and call it a day

 

[afpj]

some people described the same thing and it was HVAC system. Is the AC on when you start the engine? Just as a thought switch OFF the AC prior to shutting down, and see if this reduces/eliminates problem.

 

[06xl]

some people described the same thing and it was HVAC system. Is the AC on when you start the engine? Just as a thought switch OFF the AC prior to shutting down, and see if this reduces/eliminates problem.

I've tried it today with ac on and off and it sure did it with the ac on.thanks for the info that's explains everything when I use to hear the noise my ac was on and I started to shut everything down radio ac etc etc to listen to this while waiting 5-10 before starting and the whole time I'm thinking by waiting several seconds before starting stopped the noise and it was the damn ac

 

[KAKALAK]

My 06 Denali XL does the strangest thing at times on a cold start. Seems like every 15-20 cold starts or so, I'll get that one start up where something jolts the truck a little making a loud thumping sound right as the engine fires. There are no codes stored in the system and everything else is working normally. Transmission shifts properly and the fluid is clean. Again, it's only maybe every 15-20 cold starts that this happens. Then it wont happen for another week or two and DOES NOT happen on warm engine starts. Any ideas? Judging by the sound and feel of it, I'm guessing possibly a jolt from the tranny... But I've never seen this before.

I know its not the same year and all but I have a 2011 Silverado and I took it to the dealer for the samething. They have a TSB on this issue and said its normal operation. They said something about fluid getting by or left in a valve body in the transmission or something like that. Mine does it about everytime start it up.