i"ve got an 06 ss/sc and when i rev up the engine it sounds completely fine but as soon as i let off the gas and the rpms begin to come down i start to hear this grinding noise and my rpms flutter quite a bit
they go between 1000 and 2500 rpm a few times then it idles normally.
If u have experienced this or know someone who has and know what it may be please let me know

thanks

 

Could be the timing chain rattling. If I recall the early SC's had a problem with the timing chain tensioner, which would cause slack in the chain when theres no load.. I could be completely wrong tho, Im just pointing out a possible cause.

 

could be the hooker in the trunk......

 

thanks for tip about the tentioner
i just had it into the shop and thats what it ended up bein