Similar event to a wastegate failing open. If you retard the throttle and the noise changes (similar to misidentifying a failed engine), leave it alone. MP check will show up the issue VERY quickly and corrective action can be taken. If it is a failed open scenario, any thrust is better than a feathered engine. A wastegate failing open will still produce around 75% power at sea level in a PA-31.
Overboost is simply controlled by reducing throttle. With the Diff Pressure and Dens. Alt. controllers on a 31, they allow oil to bleed out of the system to open the wastegate. Excessively cold oil can produce an overboost (too thick and unable to leave the system). Even experienced it once, control change nowhere near the degree of a severe power loss failure, though.