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Clacking/Clunking upon cold startup, NOT the normal piston slap


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A few weeks back my truck, 2005 VHO with Magnacharger, 120,000 miles started making a rather loud clacking/clunking noise when I start it up after it sat overnight. I ran out and popped the hood but you could barely hear the noise from under the hood but it was loud under the truck and seemed to be louder on the pass side. I shut it off, restarted it and no noise. The next morning the same thing happened only this time I actually drove it down the road, about 10 yards and the noise stopped. I figured it only did it while it was cold so the following morning I called AAA, put it on a flat bed without ever starting it so I could get it to the shop cold and they could hear the noise. Of course it didnt make any noise so they never got to hear it. The next morning I recorded the noise but they couldnt tell by listining to my recording.

 

Initially I thought it was the O ring on the oil pickup tube because I was told it was a pretty common problem with the LS motors, so I replaced it and it seemed to quiet down for a couple weeks then it started again. So I took it to a shop. At first he thought it was the main bearing clearance but after putting a mechanical guage on it the guage remained stable so he does not believe that is the issue and is pretty stumped, as am I. It only does it when its cold and quiets down rather quickly but this morning it didnt do it at all, most bizarre.

 

Any ideals? Also, are there and performance shops in Southern California that might be able to help me? I figure until I at least get a diagnosis I probably shouldnt take it out of town but I do have AAA Plus so I can get it hauled just about anywhere in So Cal, within reason of course.

 

I'm going to try and attach the file of the noise that I recorded on my android. On the sound file I placed my phone under the drivers side, started it up, let it record for a minute then took my phone under the hood, as I say during the recording, then place it back under the drivers side. OK, it wont let me upload the file, its a m4a file

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I vote broken cat. I'm assuming you found nothing in your oil when you drained it to check bearings, that's a good sign. All it takes is a splash of water on a hot cat to f it up. With the pickup o-ring making a difference for a little while it makes me wonder though. Did you ever cut your oil filter open to check it? Though you can't hear it from the top of the motor, maybe a lifter has collapsed?

Does it sound like metal on metal clunk, or metal and ceramic?

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Thanks for the replies guys. I cant recall for sure if I hit some water while the cat was hot but I might have, however, wouldnt the truck throw a code if the cat is bad?

 

Live4DaThrottle, I didnt cut open the filter but I have had several shops examine and smell the oil and everyone says it looks great. Also, if a lifter collasped wouldnt it be constant? As to the sound, it kinda sounds more like sheet metal or aluminum vs solid metal, does that make sense?

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It would throw a code if it was plugged or completely gone, but if it is just slightly broke and still flows though it may not throw a code. And when it heats up it expands and stops rattling around. That is just a theory though I suppose. When you say it sounds like sheet metal it makes me think you have a heat shield loose, but you must have had it in a hoist to check everything out. Maybe check the heat shield on your starter? I could see the cat being the issue especially because of your forced induction pushing way more flow than its meant to and it goes away when it warms up. Maybe when it's cold go out with a rubber mallet or just your hand and VERY lightly give the cat some knocks, you should be able to hear the rattle if that is indeed it.

 

As for the collapsed lifter, I am leaning away from that with the sheet metal sound you explained, but my theort on that is your oil pressure might give it just enough where it stops acting collapsed.

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Magnaflow makes nice replacements. I don't have alot experience with them as that's the first thing I cut off haha but I have great experience with the rest of their products. I had them on my truck when I first got it and it performed well. Otherwise I'm sure there are other cheaper brands to go with, I just prefer magnaflow.

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The noise is sort of coming back, not as loud but its there from time to time. I've beat on the cat and it made no noise. Of course everytime I try to get the noise to duplicate for a mechanic it wont, figures huh? I've had it on a lift again and we shook the crap out of the exhaust thinking something was loose but found nothing, really sucks!

 

It isnt missing or down on power but I would really hate it if its an internal motor issue and I end up blowing up the block because its alot cheaper to rebuild it than be forced to replace it. And of course I would hate to pull the motor, get it rebuilt only to find the noise is still there. The part that bothers me the most is not knowing.

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