Odd sound from exhaust

[TJBaker57]

2005 Yukon 5.3 LM7

I have been hearing this odd sound in my exhaust for years now, and maybe 50 thousand miles or more.

Today I discovered that while the sound cannot be heard audibly at the engine bay, if I use a long metal probe placed against the passenger side head at various places and pressed lightly against my ear I can detect the sound. It seems strongest by cylinder 6. No such sound can be detected on the drivers side head.

At idle by my best estimation the frequency seems to be about 320 beats per minute. That would be close to 1/2 the engine rpm.

Anyone ever come across something like this?

2005_LM7_Yukon_Exhaust.mp3

drive.google.com

 

[Mooseman]

Almost sounds like the noise I heard in my Saab's 5.3. A light knocking noise that went away at higher RPM but at the engine. Thought it was piston slap, it eventually turned into rod knock a couple of years later.

 

[TJBaker57]

it eventually turned into rod knock a couple of years later

That's what worries me. It just seems weird for the noise to be so prominent at the exhaust outlet. That sound file is recorded at the right rear of the vehicle.

 

[Mooseman]

Have you tried the paper test at the tailpipe to see if you have a bad valve?

Maybe a compression test is in order.

 

[TJBaker57]

Have you tried the paper test at the tailpipe to see if you have a bad valve?

Maybe a compression test is in order.

I have not. I would think that after so many years and miles a bad valve would have failed noticeably by now. It can't hurt to check that. Thought i did a compression test a few years back but don't see records of it now

 

[azswiss]

The 1/2 frequency observation suggests, to me anyway, a valve or valve train issue (rocker, push rod). Bit of a long shot here; you might try comparing the characteristics (loudness, pitch, "clarity") of the sound at the head versus on the side of the block. I would think that any type of issue occurring at the crank, like a rod bearing, would not be highly isolated when listening at the head and would get both louder & clearer the closer you get to the crank. Fingers crossed.

 

[TJBaker57]

Been travelling the past few days. Yesterday evening after driving most of the day I thought to check for the noise while engine was hot. No noise at the exhaust. Didn't check for the vibration as I don't have a probe with me.