My experience with an aftermarket exhaust (an MGOC Standard S/S backbox) was that it was susceptible to a little drone at around 3000rpm. After doing some basic maths around the speed of sound, the wavelength of sound at 3000rpm (50Hz) which is 6.6m, I reckoned it was about 1/4 wavelength between exhaust pipes, if you take the measurement via the exhaust system.
So, that could form a resonator - with a reflection from the open end of either pipe reverberating back and forth between each pipe opening (oddly enough, sound reflects back both from the closed end of a pipe, and the open end of a pipe).
So, I stuffed a sock up one exhaust pipe and lo! The drone disappeared. Until the sock blew out.
All of which is by way of saying, I think you actually could have a worthwhile effect on any resonant drone, although I'm not sure if a hard surface on your valve would work. It might - it will reflect sound too, but in the opposite phase to an open pipe, which may have a cancelling effect. Worth a try.