Light Rattle at idle and 1500RPM

[1998Suburban]

I've been trying to track down a quiet, low frequency rattle on my '98 L31 K1500 Suburban but haven't been able to pin it down. It happens at 2 speeds: idle and 1500-1800RPM with light throttle. It only occurs about 50% of the time at idle (1 of 2 stops at a red light). At speed under light throttle it's more consistent.
It's quiet enough that, with the radio at low to moderate volume it is unnoticeable. I've done some research and it seems like there are a few possibilities: catalytic converter starting to come apart, rattling O2 sensor, exhaust making contact (looked but didn't see anything), or a wrist pin rattle.
Has anyone experienced a rattling O2 sensor? It's apparently common on the LSx engines. Also has anyone experienced a wrist pin rattle with the engine warmed up? Engine temp doesn't seem to make a difference in this noise.

 

[laredo1307]

It's not anything in your exhaust coming apart. I chased this same issue for quite a while. Do you notice that your radiator overflow gets low but there's no sign of a leak anywhere? If so you're probably suffering from the same thing my truck is. Basically the gasket is failing on you're intake manifold near the rear of the engine. Water is getting into the combustion area and that's what's causing the ping-ing sound. They have an uprated gasket set that you can purchase to fix this issue. I have it planned for this spring when I have some more free time.

http://www.tahoeforum.com/showthread.php?t=525761

 

[2000tahoeZ71]

If your going to put it off, keep an eye on your oil. Water in the oil is BAD for bearings. I just did the job on my truck, and replaced the spiders while I was in there.

 

[laredo1307]

Yeah, my oil is clean and my exhaust seems normal. I haven't seen any signs of antifreeze getting into the engine.

 

[2000tahoeZ71]

Just Keep an eye on it. I'm pretty sure my cats got screwed up from burning antifreeze. About $ 1000 later I'm back in business.

 

[kcb37]

Yep, burning anything but gas and air will destroy a cat.

 

[1998Suburban]

I would have never equated a leaky intake manifold gasket with that noise. In my case, however, I'm pretty sure that isn't the cause. My LIMG sprouted an external leak near cyl #2 in April 2012 and I replaced them with the Fel-Pro problem solver gaskets. The engine wasn't contaminated internally as I caught it early on. I also did the CSFI to MPFI conversion with the upgraded spider and injectors while I had it apart. I haven't lost any coolant since replacing a leaky radiator last Feb.