My 2.8 V6 '88 Blazer Sport is making a rattling sound ( like a marble or how do I explain it ) while I am standing still with autotrans in "D". It goes away when I put him in neutral or park or when starting driving again.

Any toughts ? It drives and shifts like it should and I cannot reproduce the sound whil upping the RPM while in neutral, maybe a little.

Anyway it's really loud under the hood.

 

A couple places to look

take the dust cover off the torque converter
if its louder take a look at the flex plate / flywheel
I have seen the flex plate crack all the way around the bolt holes (SBC) where it bolts to the crank shaft

Also take a look on the bottom of the catalytic converter for a plug , old GMs has a plug that would rot away and rattle
Some times the inside of the catalytic converters also rattled loose

Check your fan blades for cracks , engine off
NEVER stand inline with the spinning fan blade while revving engine

 

Mine does this, I would put money that it is your exhaust.

 

Considering what I can hear ... well the noise is always there, but the loudest @ 1300 rpm and that happens to be when holding the brakes while in D or R ... but I can reproduce it while in N or P. If I had to pinpoint the sound it would be my headers producing it.

Considering the fact it runs and shifts great, has only 35k miles and what I read about the internals of the cat coming loose I hope it will be as simple as gutting it to get rid of the noise.

My Blazer runs on propane, so exhaust heat is a lot higher what could explain the cat going bad. I also get to rich and to poor mixture engine codes since it runs on propane and last weeks it seems to be more thirsty.

The only thing is, the cat does not seem to be making a lot of noise, it is loud under the hood. Marbles in a metal can would be the best way to describe. So the headers act like speakers for the cat ? I certainly hope so, that would be an easy fix.

 

Just found the culprit because it got louder ... It appeared that the bolts of the bracket used to bolt the steering pump to the engine loosened.