Engine knock from exhaust when cold

[Therealsmitty]

Hello all, and happy holidays! I could use some help here please;

I recently purchased a 2003 Yukon XL with 140k on the clock, 5.3l engine.
I was a little rushed after looking at many Tahoes/Yukons, especially since my wife was due with our 3rd child in less than 2 weeks. I made one sinful mistake in my purchase... When I went to look at the Yukon it was warm already, supposedly because they needed to put gas in it. All checked out; very very clean, etc. I got cash and picked it up about 4 hours later and started it again somewhat "cold". Again, all was fine.
After sitting for a few days at my house, I started it again. This time I noticed a hollow knock sound coming from the exhaust. Stock exhaust mind you. I have another 5.3l truck that does not make this sound. When you first start it cold, it smells a bit of unburnt fuel, though only for a minute; could be normal. The knock eventually goes away, but it takes a good 10 min. It's a hollow sounding knock, but again, you CANNOT year it in the engine compartment or in the wheel wells or under the truck, only from the exhaust. It doesn't get louder if I rev the engine... might even go away when I do in fact.
Oil was changed by last owner about 2k miles ago with conventional.
Oh, it doesn't take long to turn over, but it doesn't spring to life either the way my other 5.3l does.
Thoughts? I can't seem to nail this one down with a Google search.
Thanks all!

 

[drakon543]

junk plugs? used a conventional oil that was too heavy for the upcoming cold weather. 2k on the oil for conventional oil i dont like to break 3k with regular oil so i would go ahead and do an oil change. checked for anything that doesn't look like it should be in the oil. it could really just need a solid tune up and some good run time. previous owner could of just had it sitting around alot until you bought it.

 

[Big Mama]

Agreed no knock should come out of anywhere start with oil and check plugs

 

[Therealsmitty]

Thanks guys. First thing I'm going to try is a full synthetic 0w-30 just to see if the low cold temp viscosity makes a difference; I would suspect it will. I will update in a few days assuming my local Walmart carries it, otherwise I'll just try the 5w-30 synthetic.
Cheers

 

[SnowDrifter]

I use 0w30 myself. Superior oil IMO.

Could you post a video of the noise?


Piston slap is common


Could be a cold misfire. Are you able to check live data?

Could be something in or around the exhaust itself. Crawl underneath and start tapping with a rubber mallet


Is the knock rpm sensitive?


Have you verified its from the exhaust and not something under the car? A cracked flex plate comes to mind

 

[02Lightning]

Sounds like piston slap to me.

 

[Therealsmitty]

I'll see if I can capture it on video. As far as piston slap is concerned, I was always under the impression that only lasted for a minute or less, not 10-15 like this sound. Is that not right?

 

[Therealsmitty]

Here's a short clip of the sound. Again, you can strictly hear this at the exhaust, nowhere else. And it DOES speed up with the engine rpm's.

 

[iamdub]

Sounds like a mild performance muffler to me. Have you looked at the muffler?

 

[Doubeleive]

ya I don't hear anything bad in that video, my 03 Silverado has a terrible exhaust leak/piston slap or something for the first 2-3 seconds of a cold start and then it's gone until the next day makes it very hard to track down and it's been that way for a few years and hasn't gotten any worse so I just kinda live with it.