Propeller noise is highly sensitive to tip speed (specifically tip relative Mach number), but exhaust noise is only approximately linear with power output. So as you go up in RPM for a given prop diameter the propeller noise will eventually dominate exhaust noise.
If you have a constant speed prop, then using a lower RPM and higher MP will generally be quieter, and a 3 or 4 bladed prop of smaller diameter will likewise be quieter at the expense of reduced static thrust.
In Germany, where the noise regs are more severe, you see a lot of exhaust silencers and 4 bladed props.
Balanced exhaust systems seem to improve power output, reduce noise and reduce shock cooling problems - its a pity they also weigh more.