2000 Z28 - Bottle noise coming from the back, 'somewhere'

Hello all,     In my 2000 Z28 I have started hearing what sounds like: 'A pen being rattled inside of an empty coke bottle' coming from the back end when
the car is coasting (under 30mph). For example, letting off the gas before turning onto a street. 2 weeks ago I changed the rear brake pads and since I had the wheels off deceiced to check around. I have checked the suspension, cat back system, rear fold down seat, took out the factory jack and still cannot find what is making this noise. Took it to the dealer who said it may be bearings (right), which they packed and made no difference.
Was wondering if any other camaro owner of similar year has heard any noise like that when coasting?
Any advice is greatly appreciated! Thanks
(The only modifications I have done is the Flowmaster cat back and the SLP cold air induction kit)
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Have you checked for loose heat shields in the exhaust?
See if alldata.com has any bulletins listed under "recalls and bulletins" for this issue. Joe--ASE Certified Parts Specialist & 10th Ann.Club Tech Director '80 Carousel Red Turbo T/A, 26k orig. '79 "Y89" 400/4 speed 10th Ann. T/A, 57k orig '84 Olds 88 Royale Bgm 2 dr, 307 "Rocket" (lol), 141k and still going.... '80 T/A project car...
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