No boiler guys answered yet?
The standard setting is 12 lbs cold. 1 PSI will raise water in a column 2.31 feet.
The pressure should be set to what is required in total height of system. For example, if a boiler is in the basement, and its a 2 story house with baseboard radiation on the second floor, it could be 8' + 9' + 1' = 18' (boiler to upper baseboard).
1 PSI / 2.31 = 0.4329
18' x 0.4329 = 7.7922 minimum PSI required cold temp
With pumped applications because of circulators will see a pressure differential across the impeller, its advised to adjust for this. Industry leaders agree on 3 PSI for residential size circulators.
Thus 7.8 PSI + 3 PSI = 11 PSI required (12 PSI is a industry chosen number perhaps because of what if its radiators that are taller on the second floor)
So 12 PSI is a starting point but not necessarily correct for all applications, for example a 3 story house likely requires a higher setting.
The second part of your concern/question-
Water expands as heated and on a closed system whether its pushing against a rubber bladder with a air charge behind it or a direct expansion tank charged with air it will increase in pressure as it is heated and expands.
There is no magic number on what it should be as its different depending on the cold start pressure, the height of heating system, the type and size of expansion tank and the operating water temperature.
To give you an average operating pressure, a two-story house, operated at 180 degrees, with baseboard, with a properly sized bladder type expansion tank will go from 12 PSI cold to 19-24 PSI when hot. This is well under the residential boiler relief valve usually set to blow off at 30 PSI.
If the gauge is accurate, the expansion tank is sized correctly and you are still hearing noise with proper air elimination installed in the correct location, try increasing the cold pressure an additional 2-3 PSI.
Increased water pressure does not increase heat output. However if the pressure is low and circulation is not taking place properly there could be reduced heat output delivered.
The operating temp of the boiler should be matched to the structure heat loss/circulation flow rate/heat emitter design and output. These are all related and affect each other.