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Discussion Starter #1 (Edited)
So the vibration started early September, approximately 1-1/2 month after I installed Kook's headers with hi-flow cats... So, this is how it's been presenting itself, which basically is my daily commute - Nothing at cold start-up, car is in Tour mode so I don't wake up the neighborhood. Drive a few miles at city speed limits, ~45 mph, before getting onto the interstate. I've switched to Sport mode by now. As I'm accelerating to interstate speed, which I'm guessing is 80... It's during the acceleration is where the vibration noise presents itself. As an example, it sounds almost as if a lug nut was setting in the wheel well and with exception of the lug nut, the wheel well was empty and the lug nut was being vibrated by the resonation of the exhaust. However, that isn't the case, I do have a spare tire, properly secured. But to eliminate that as a possibility, I removed everything from the trunk, except the battery. Still happening...
It's not restricted to only when the car is accelerating, once the noise starts, it will randomly keep "vibrating" at idle, out of gear, foot off clutch. What's odd is that when I can get it to continue while parked, I can't hear it underneath near the mufflers, but open the trunk and you can hear it.
Another oddity is that while on the interstate, if I accelerate normally from ~65mph while in 6th gear, it sounds like a Z71 truck coming up on my right side. So it's basically a mix of my exhaust and what is perceived as another deep tone exhaust.

Since everything remaining in the trunk area feels secured and no loose objects have been discovered, my next thought was that an exhaust clamp wasn't properly tightened and it worked loose. So I QuickJacked up the car, checked all of the clamps, heat shields, license plate, axels, etc... All seemed solid.
Started the car and got it up to temperature, letting it idle long enough to get the exhausts too hot to touch. Revved it several times and couldn't get it to act up. Had my girlfriend put it in gear and slowly accelerate to see if it might be coming from the differential. All sounded normal so I had the GF shut 'er down....
Then as I stood up, I looked down and saw a condensation spot on the ground underneath the tips of the driver side exhaust. Eh, that's normal... Walked around the passenger side, look down and the condensation spot for that side was closer to the tire. I thought "self, that ain't right...". Got back underneath and noticed that the condensation was dripping from the seam on the inlet side of the muffler. See picture...

So I'm guessing this is my culprit, but since I'm no expert in this area, I thought I'd get some input from a few of you in the forum to see if water coming from this spot is expected.
Also, are the stock bi-modal mufflers capable of handling the extra exhaust flow from the aftermarket setup? I have an extended warranty (non-GM), so it'd be good to know if they could factually assign blame to the performance mod before I try to have it replaced under warranty.

I'm going to do some troubleshooting tomorrow specifically to that muffler to either identify or eliminate it as the culprit.

BTW, the picture of the exhaust system is back in July, when I installed it. The water drip is from today.
 

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Discussion Starter #3

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1- exhaust is not covered under 99.99% of any extended warranties. Read your contract.

2- take off license plate and see if it goes away. Also check rear fascia and reflectors for looseness.
 

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Are the spacers still between the exhaust and the tunnel brace? My exhaust had dropped just a tad a couple of weeks ago and was vibrating a bit on the brace. Easy fix for me, hope you diagnose and find an easy fix for you as well. (y)
 

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Discussion Starter #6
1- exhaust is not covered under 99.99% of any extended warranties. Read your contract.

2- take off license plate and see if it goes away. Also check rear fascia and reflectors for looseness.
@CB750
1 - You right!
2 - Removed the rear fascia... While removing it I noticed that the wire harness was hanging down and close enough to come in contact with the heat shield. I'll have to reference one of my other cars to see where the retainer is supposed to be. The SS was involved in a minor accident that supposedly caused <$500 in cosmetic damage to the rear fascia. I knew this going into the purchase as it was on the Carfax. Whoever did the bodywork also broke the locking red tab on the wiring harness. This kind of inferior work is likely why the costs were kept under $500.
The reflectors and others items attached to the fascia are all securely attached with no play.

@jalanrr The gap between the bracket and muffler on the driver side is really close, but not touching. There's no evidence of the muffler and bracket coming into contact with each other. I tried sliding the pipe that connects to the X-pipe and to the muffler, up to increase the gap, but the flare isn't deep enough. I'll either have to have the flare extended, or stuff something between the muffler and bracket. I'm going to opt for the latter, however this doesn't solve the problem that will now plague me of knowing that the driver side muffler extends out approx 1/4" further than the passenger side.

I'll take care of these 2 items separately, testing for the vibration noise between patches so I'll know which one was the culprit. If either.
 

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Hopefully it is one of those. If not, you could use an action camera to help find the rattle. There are some decent inexpensive ones on amazon. Or you could go the old school route and stuff someone in the trunk. This might help you to isolate it a bit more.
 

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Discussion Starter #8
Problem solved!
It was the left muffler vibrating against the bracket. Got lucky and the vibration continued as I parked the car. Again, it seemed louder in the trunk rather than underneath right at the muffler. I weighted down the muffler by putting my foot on the tips and the vibration stopped. 😃
Removed the pipe that connects the muffler to the X-pipe, took it to a local muffler shop and had the flare widened (his word) so the pipe could be pushed further onto the X-pipe. This allowed the muffler to move forward extending the gap between the muffler & bracket and also allowing both sides to match in alignment.
Tested it by driving the same route I do every day and not a peep from the annoying vibration.

Thanks to all of you for your input, along with Armin Defiesta's thread for helping solve this annoying problem.
 

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Definitely a muffler bearing.
 
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