Guidelines for using a constant speed/adjustable pitch propeller
why would anyone want to mess with the pitch of the propeller ?


A propeller runs at its highest efficiency when used at a particular combination of forward speed and RPM. By changing pitch, you change this combination, so the best efficiency can be shifted over a range of airspeeds.

For the physical background please consult this answer.


  This implies that the lower RPM will result in lower thrust ( true? )


Yes, lower than maximum thrust. If you need less than maximum thrust, a lower RPM is sensible to reduce engine wear. Reduce RPM only if the remaining prop speed can still absorb the power; in other words, make sure the RPM is high enough to allow the engine to run at the speed needed for the desired power. But do not control thrust with RPM - this is what the throttle is for.

A piston engine outputs a constant amount of power, regardless of airspeed. This power output grows about linearly with engine RPM. You control this power with the throttle (and measure the power indirectly with the manifold pressure) and use the prop pitch to allow the engine to run fast enough to create the desired power at all. If you run the prop faster by reducing pitch, it will operate at a blade lift coefficient which is lower than that of optimum blade L/D, so you lose efficiency. Also, the engine will run faster and be in the partial-load operational range where it also runs less efficiently. Run the prop slower by increasing pitch and you will slow down the engine so it cannot produce the desired power. Also, now the propeller will run at a higher lift coefficient, efficiency will drop and part of the prop blade might even stall if you increase pitch too much.

As to what is a significant reduction in noise: This is subjective and I advise you to find out for yourself.