when im kinda idleing around 5-20 mph the gmmg makes a small noise that sounds like a rattling heat shield, but after 25 it doesnt do it, its not touching anything underneath.

 

well, have you checked all your heat shields?

beyond that it could just be that specific frequency that causes a more noticable vibration with the muffler. that can be very "car specific"

i wouldn't worry too much as long as you know it isn't something actually rubbing

 

Mine does it too when idleing........Going 20mph I can't really hear anything but rumble anyway...if everything it tight and not moving around, I wouldn't worry about it.

 

The noise stops after it has been on for awhile. If you take it off for any reason, it will do it again. It is just a settling in thing.

Just try to keep in mind that GM bulit these cars for use on the street by the average driver. All parts that came on the car are matched to withstand ONLY that use. Therefore anything that is changed will have some reaction on another part. Changing the internal harmonics of the muffler just created a frequency that resonates in a range that the body insulation can't filter out. Another example would be putting a short stick on your shifter. That creates extra effort to put it gear since you just lost a significant amount of leverage. If that stick eliminates the factory rubber insolator, you get extra gear noise in the car and your soft aluminum shift forks are no longer protected.

It goes on and on. Before you make changes, try to figure out what else will be involved with that part. A good way to do this, is to use the search function. I used to lurk on this board and use the search function for a long time before I registered. By researching before i made changes to my LT1 nd now my LS1, I had agood idea of what worked and what was a waste of time and money. For instance, I bought a Holley Power shot over a K&N because the oil is sensor friendly, while K&N's is not. The Holley also seals better. That was discussed here along time before GM started refusing warranty work because of over oiled reuseable filters. I only use the filter for shows because it looks pretty under a clear lid. Otherwise, I use a paper filter and change it often because it filters better and makes more horsepower. Pyshcocabbage has done real dyno testing and has numbers to back it up. Also, BOBTHEOILGUY, has tons of info on this and oil filters on his site. All this was found using the search function on this site. Other sites have disclaimers on the top of every forum that say that you should do a search before posting and that duplicate posts will be deleted without notice. This site is much busier than any of those, so, that is why so many get through. It is very irritating when you are trying to ask a question about something that you can not find using the search, but you have to wait to see your question posted. It is waiting its turn because things like, which lid to get is being asked and discussed for the enith time.

Sorry this is so long. DaveD, feel free to edit.

PJ