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Try a search for the rear end pop/tink on SVTPerformance. Sounds like you may have it. Chicago Mike and I discussed this issue years ago on SVTP before ChicagoSVT was born.
If not that, it could be the body panel rubbing problem or some other rear suspension problem but it sounds like pop/tink from the way you are describing the noise.
Actually if it's not the tink... because the tink is caused by the axle spines missing, and it can be recreated in 1st and reverse...
Check the catback, sometimes the pipes are mounted too high and bounce off the fuel tank cover or elsewhere, inspect for marks of this happening. otherwise if it's not this or the tink, have the rear suspension inspected. Check wheel bearings, suspension parts.