Can you think of anything that may be related to, or caused by, the bolt tightening? If the sound started upon bolt tightening, something else had to happen because a tightening of bolts (by itself) won't do that. What else could have been part of the bolt tightening process?

Cold starts can sound different.
Could anything downstream on the exhaust have been affected in any way?

Also see if you can isolate the location of the noise. That will say a lot about what the solution is.

I'm pretty sure that the fact that because I had to force the socket down into where the bolt is that that damn heat shield is rattling ... I just didn't think it would do it only under engine load.

Well, max engine load will naturally have more vibration.

Plus - The engine only twists in the engine bay while under load. The clearances also change under load.

Of course... I just really think it must be a heat shield rubbing somewhere, but I don't know... it's pretty frustrating. I should just take it to a shop and let them find it, but I just don't have the money for that so i'll have to keep checking. I don't suppose I could take the heat shields off completely huh? That would almost certainly get rid of it :-)

UPDATE: My roomate got under the passenger side of the car tonight and heard the rattle. He pushed and moved around the heat shields and there was no change in the rattle. He put his hand on the pipe and said when it made the rattle noise that he could feel it in the pipe. I'm not concerned about it now as I know some people have had this noise while the pipes are breaking it.

I had wondered why it took so long for them to start making the noise, but I honestly drive my car very gently the majority of the time so maybe it's taken it a while to get the pipes through their cycles.

Tony, do you recommend a few WOT runs to 60 mph or anything specific to help with this cycle or does it just take mileage and constant heat?

1500 miles of normal varied driving.

oh ok, well i'm glad that's what it seems most likely to be since I expected this much earlier... but at least now I know it's just the pipes and it will clear out... i'll just clinky dink around for a few hundred miles and be happy :tup:

Well update again. The noise is almost gone completely. Once it got hot and I got on it a little bit just a few times... nothing over about 4k, the sound could barely be heard. I got onto a road with guardrails where I can always hear the sound... nothing though. I got home where there is a huge concrete wall and no sound. Later yesterday I started it up and there was just a very faint sound, maybe because the car had cooled off for a few hours. Either way though... not anything I expect to be an issue. For all I know when I tightened down the bolts a very small piece of rust or anything could have come lose and gotten into the ART pipe? I don't know. Anyway, getting better, more broken in and maybe a little more power? Could be in my head there though lol.