08 4.3 exhaust rumble !?

[NONHOG]

Anyone have this issue ? Just curious , my Silverado "rumbles" during warm up just about when its fully warm(under load, part throttle) . 1st time I heard it I thought a semi truck was right next to me doing a jake brake.
Had it in the dealer for a couple of other warranty concerns and mentioned it.
They did not hear it but put in a section of flex before the muffler.
Because there was a TSB on it.
Now its more frequent but not as loud . Time for aftermarket cat back ?
Hoping for a cure .