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Loud bang when cooling down

SierraSmurf

By SierraSmurf
in Ask A GM Technician

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SierraSmurf Enthusiast

SierraSmurf

Posted
Posted

I have an '05 GMC 2500 HD with the 8.1L and Allison tranny. About 2 or 3 minutes after I shut down the engine, there is a VERY loud metallic "bang" coming from under the hood. I assume that there is metal cooling and contracting somewhere which is causing this noise, possibly the exhaust manifold covers, but have no idea what they would be catching on. Any ideas? This is driving me crazy. It doesn't do it after short trips, only after it's really good and warmed up. Thanks in advance for any light you can shed on this!

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted

Are you positive it's from under the hood? We have an issue with the exhaust systems doing this. There's a TSB to cut off the muffler shield and put a new one on with clamps, though I'm not sure if it applies to an '05 full-size. I'll have to look it up.

 

My '05 Colorado did that for the first thousand miles or so. My garage is just off the family room and after I'd pull in to the garage and go inside, I'd be sitting in the family room and I could hear it popping and banging out in the garage for 15 minutes after I parked it. I have 3000 miles on it now and I don't notice it anymore. It never bothered me because I knew what it was and I wasn't about to go chiseling off the heat shield.

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted

Here's the first part of that bulletin I mentioned. Note that it says it does NOT apply to the 8.1. I don't know why.

 

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Popping/Snapping Noise When Vehicle is Hot, at Idle or Immediately After Vehicle is Shut Off (Replace Muffler Heat Shield) #03-06-05-008B - (Jan 13, 2005)

Popping/Snapping Noise When Vehicle is Hot, at Idle or Immediately After Vehicle is Shut Off (Replace Muffler Heat Shield)

2002-2005 Cadillac Escalade, Escalade EXT

 

2003-2005 Cadillac Escalade ESV

 

2002-2005 Chevrolet Avalanche, Suburban, Tahoe

 

2002-2005 GMC Yukon, Yukon Denali, Yukon Denali XL

 

2003-2005 HUMMER H2

 

with 4.8L, 5.3L or 6.0L Gasoline Engine (VINs N, U, T, V, Z -- RPOs LQ9, LQ4, LM7, LR4, L59)

 

This Bulletin DOES NOT APPLY 8.1L vehicles at this time.

 

This bulletin is being revised to include additional models and parts availability. Please discard Advanced Service Information Bulletin Number 03-06-05-008A (Section 06 -- Engine/Propulsion System).

 

Condition

Some customers may comment on a popping/snapping noise from underneath the vehicle at idle or immediately after the vehicle is shut off. Some popping/snapping noise from the exhaust system during cool down is a normal condition.

 

The noise may be intermittent depending on the outside temperature and or temperature of the exhaust system at the time of testing.

 

Cause

As the exhaust system warms and cools, the muffler and the muffler heat shield expand and contract at different rates and may cause a popping and/or snapping noise.

 

Some popping/snapping noise from the exhaust system during cool down is a normal condition. The customer should be made aware that this service procedure is designed to reduce but not eliminate all exhaust system popping and snapping during cool down.

SierraSmurf Enthusiast

SierraSmurf

Posted
  • Author
Posted

I'm pretty sure it's from under the hood, but I'll try to isolate the location next time. It doesn't do it every time, but when it does, it's not a popping sound, only one loud bang. A heat shield was my idea as well, since the engine is shut off, but I was thinking it's the one over the exhaust manifold. I would take it to a dealer, but I'm not sure if I could re-create the sound. This only happened 3 or 4 times and it has been recently. I've got 3200 miles on it now and it never did it until I had over 2000 miles on it. Thanks for trying though! I guess it's safe to say that this isn't a "normal" noise for this engine...

Trouttrooper Enthusiast

Trouttrooper

Posted
Posted

Interesting that it specifically does not apply to the 8.1 When I turn off my truck it pops, bangs, and cracks for about 10-15 minutes. I know it's just heat shields and such cooling down so I'm not too concerned. I have no neighbors so at least it's not bothering anyone else.

SierraSmurf Enthusiast

SierraSmurf

Posted
  • Author
Posted

Perhaps GM is evaluating the issue with the 8.1 and will come out with a separate TSB for that engine. I wouldn't normally be concerned with a series of pops or small bangs, but this is just one... and it's loud. I mean like sledgehammer on metal loud. Now I also hear random rattles coming from under the hood as well. It sounds like a Duramax sometimes. I don't believe it's the piston-slap issue, since this is while I'm driving and its already warmed up to operating temp. Guess it's time to find a dealer in this area. I purchased the truck 3 states away when I was visiting my parents and just moved. I don't know much about the area so this should be the icing on the cake! Anyone know of a good service department in north-western Washington (Burlington/Mt. Vernon/Oak Harbor)?

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