True or false: CWA ballasts are humming more loudly then reactor ballasts?
I am asking this as i saw several fixtures (Mostly high and low bays) with US probestarts NA-SC MH lamps (Mostly GE) that hummed extremely loudly and i have never saw the same type of fixtures but with european MH lamps do that.

Series choke ballast current waveform is close to sine wave, so it only hums at the base frequency (50/100Hz) and higher harmonics can only be excited by some loosely vibrating material, while I believe the current waveform of a CWA ballast looks like "spikes" pointing up and down, so it has the higher harmonics already present...

CWA uses core saturation to regulate current, so there is way more flux with high harmonic content in the surrounding area, what causes other metallic (mainly ferromagnetic) parts to vibrate (e.g. ballast holding straps,...)
HX or series choke are designed so, they never saturate, so so both magnetostriction and the vibration with other components is much smaller. But some of them (mainly series chokes) do vibrate during lamp warmup, as there is ~15% higher voltage on them, when the lamp is cold, what causes slight saturation.

It can depend on physical construction. I've had reactor ballasts become extremely loud with age. Some become loud after only a year. The coil becomes loose from the iron core, but can easily be fixed with class H transformer varnish...then they are quiet like new again. I've noticed that CW ballasts are harder to make quieter, however.