Engine clunking sound at startup

[azfitch]

I originally posted this in the wrong section, but anyways.....




I have a weird clunking sound coming from my 05 5.3 tahoe at startup. It only clunks after the truck has been sitting for a while. Ive tried turning off the a/c on startup and it still makes the same sound. It normally clunks about a .5 second after startup and is very brief. The noise only happens once, just after startup, and it's only for a split second.Any ideas?

I found this explanation of the problem in athother thread in the older model section:

"In that same instant there is a sound coming right behind the middle of the dash (near the engine) that is very similar to someone popping a dent out. Imagine someone popping a dent out of a Rubbermaid garbage and that would be a sound similar to the one I hear. I wish there was another way I could describe the noise but that's what it sounds like to me."

 

[Sepiroth]

Does it do it on the very first start up of the day or everytime you start the truck eventhough it sit for 5 minutes?
The sound you describe to me sounds as if it is the infamous cold start knock.

This is a very well known issue with these engines. It doesn't seem to be a big issue as there are many of these engine with well over 175K with this sound still running strong.

 

[M.M. 700]

Next time before you shut the tahoe off. Completely turn of the front and rear air/heater controls. See if that helps.

It sounds like a flap or something in the venting/heating/cooling to me.

 

[oneradride]

You might be on to something there… There is a large vacuum actuated flap (door) on the passenger side just under the cowl area for the fresh air intake. There is also a one way holding valve in that line somewhere because after you turn the truck off it will remain open. If the valve has gone bad it may be that door slamming open?

 

[03HoeRider]

I have the same problem with mine. It doesn't do it everytime, just sometimes when I initially start it.

 

[azfitch]

Thanks for the responses........It definately only clunks after the truck has been sitting for a while. Ive tried turning off all the a/c controls before starting and that does not work. I dont think it is the actuator doors for the a/c inside the cab as you can definately hear the sound coming from the engine bay. It almost sounds like a cluth engaging. Is there a clutch on the alternator or the a/c compressor? I wonder if it is one of those components failing?????