Go to any BMW dealer and see if you can get some used mufflers off a 335 (or other BMW, but these are common). They have a valve after the muffler. They can rattle at certain engine RPM's. Some customers don't notice, some do. The ones that complain get new mufflers, meaning they will be throwing out lots of old ones.
They are operated off a spring loaded vacuum actuated diaphragm. Full vacuum closes the valve, no vacuum opens. It would be very easy to use a PWM solenoid (like an EGR solenoid) to control the vacuum, and therefore the amount it's open.
Much easier, IMO, than using a motor and having to figure out where the motor is at.