I've just installed my warranty replacement Gibson dual axle-back exhaust because my first one developed a rattle inside. Now, the new one is doing the exact same thing.
Very odd, its more like a harmonic at a very distinct RPM (manual trans) - about 2500rpm there is a rattle as the RPM's just pass it. Its quick, and there every time I accelerate past that RPM - only for a very short second.
Originally, I thought maybe it was hitting something on engine flex etc...but my exhaust shop and myself had a very good look and there is absolutely nothing the exhaust is torquing against. Nothing loose on my hitch, tips etc.
While the first one was away for warranty, I threw on a Flowmaster 40 and the noise was completely gone. Get the new Gibson back, slide it on and the same noise again.

The only thing I can think of is maybe these exhausts don't like the extra air-flow through them that might come from a smaller than stock resonator. I've put on a short Magnaflow resonator and I'm sure the air going through the exhaust is probably doing so quicker now and maybe there is some baffle inside the exhaust that doesn't like it. Its about all I can think of. In a couple weeks I'll be putting on a bit larger resonator to quieten it down just a bit so time will tell. Reasoning behind this, is for short bit, I ran with no resonator and the rattle was ALOT louder.

Anyway, just wondered if anyone has experienced this type of thing or more specifically with a Gibson dual. I never expected it to come back again with a replacement muffle.

thanks for any insight