Two questions (170 engine)

Well, we had a pretty nice day for a test drive today... I believe the noise is now almost definitely a sympathetic resonance within the exhaust system, It will still appear at approx. 45mph and will dissipate when throttle is let-off. I believe the noise occurs at a specific range of rpm. I can duplicate the noise with the engine in 2nd gear at a lower mph, and also in neutral at a specific rpm range.

OK, now what? Anything to do without replacing the entire exhaust system?
Turn radio volume up? Noise cancelling earphones?