Q2: Why does my 4xxx unit make a buzzing sound when powered by AC voltage?
Answer: Certain classes of ceramic capacitors exhibit a normal characteristic, called piezoelectricity.
In some cases, an acoustic sound may be heard, coming from the capacitor itself. Ceramic piezo effects are well known, and were even the basis for the ceramic phono cartridges used in the past. This is a normal characteristic of the capacitors used in the 4xxx modules, and has no affect on reliability or accuracy of the unit.