[CrustyNoodle]

From playing around with the "golf tee" trick (unplugging the wire to the solenoid valve does the same thing and is less of a hack) I found that the valve seems to be there to reduce an exhaust resonance around the 2,000 - 2,500 rpm mark (I didn't log it, I'm just going by ear). It seems as though the valve is only really closed around these engine speeds when on part throttle which means that it would be a perfect thing to add to an after market exhaust (with some additional plumbing) to reduce the annoying resonance whilst allowing the desired sound when you want it.