Engine resonance at 2,800-3,000 rpm

[Uwon]

I have a 997 C2S with an exhaust resonance that only occurs when cruising at 2,800-3,000 rpm. It can best be described as a rattle. It does not occur under acceleration nor decel. My primary dealer says that this is normal for 3.8's. Another dealer has not been able to pinpoint the issue any further. Has anybody else had this issue? Welcome any thought and ideas.
Thanks

[UNTERBERGAMOTTE]

Do you have stock Exhaust ?

[Uwon]

Exhaust is stock. I should add that the resonance has always been there at the same noise level and pitch (40,000 km).