it's a rare thing now a days that I come across an old magnetic ballast from the mid to early 70s back.  When I do, almost always there is the soft distinctive "buzz" I must admit, I like hearing it.  Anyone else? 

I hear it with Chieepeese electronic ballast as well - it is NPF (so no power factor correction in the rectifier) ballast, where the input EMC filter choke is placed before the rectifier, so subject to higher value narrow current pulses. As the coil is likely rated "just on the edge" for the rms current flowing there (it run very hot), it is saturated by the current peaks, what cause magnetostriction, so the buzzz. But even when the coil is small (10x12mm), it make same noise as 30year old preheat choke for the same lamp...

I dont mind be buzzing 60hz ballast. at least you know it not the crapy stuff. When your in a room with magnetic ballast there always buzzing louder the rest. i have some i am expermenting with high CPS transformers as a choke. imagine what would 400Hz would sound like on a preheater. even what really bad seeing bunch of floros. running at 25Hz on 3 phase the place look like a fast chaser set!

I have a bunch if vintage Israeli switch start ballasts from the 60's / early 70's

The common part of them all is that the core has huge air gaps (it is open core), and the I-shaped plates are just stacked in there with very little potting, so they usuall buzz quite loudly. Also the buzz changes or disappears as they warm up (and expand and take up the space so they can't vibrate)

They also have different sound for preheat (pure sine) and working (not really sine due to the lamp distorting the wave)

Modern ballasts use closed core and are vacuum ipgregnated with epoxy which makes them completely silent

They also have different sound for preheat (pure sine) and working (not really sine due to the lamp distorting the wave)

I would rather guess the normal operation would be way closer to the sine then the preheat, as during preheat the core slightly saturate, "stretching" the current peaks up, so form higher and narrower pulses (with higher harmonic content).

Preheat : "Ummmmmmmmmmm"
Work : "Tzzzzzzzzzz"

The core is open (ie coil is wound on a big stack of I's then padded with some more I's on either side), would it saturate ?

Why it should this be different from other designs?

The difference in buzz/hum could have many reasons:
Either the current shape
Magnetostriction (follow the current, happen mainly when some part saturate) vs mechanical rattling (when something is loose)
Temperature and related mechanical play between some components
Or the average force squeeze out some play
...

The tube voltage shape would not play any role, as the ballast current (so forces) is an integration of the voltage. And the triangle (result from integrating a rectangular wave) subtracted from a sine three to four times larger have very little harmonic content, so if the current shape is the cause, it won't differ from the sine (taking into account other nonlinearities).

I neither like or dislike the buzz from magnetic ballasts, but my parents used to have an electronic ballasted CFL that made an extremely annoying high frequency noise. It wouldn't always, but sometimes, and it slowly varied in frequency. Not very loud, but still unbearable so I replaced it. It was probably an 11 W Ikea CFL. Except for the noise, it worked normally.

It is strange, selfballasted CFL's usually work way above 20kHz (usually 30..50kHz), so it should have been some subharmonic oscillation, what should not be there at all.

The inverter (ballast) of my notebook's screen backlight used to make a not-very-high pitch HF sound, that would change in loudness and frequency in different brightness settings. Somehow it is way more silent today

Most emergency lights (Royer or blocking oscillator style running on NiCd's) have a pronounced HF sound of 10<X<20 KHz, which is also unstable and changes as the battery is depleting

Here is the magnetic ballast in question

Notebook ballast: The main inverter operate at high 10's of kHz, but it is interrupted by PWM (and/or PDM) at 100Hz..few kHz for the brightness control. And I guess this starting and stopping PWM is, what make the noise, not the ballast steady operation.

Buzz:
I had a look to to your ballast picture: I think most of he noise would come from the metal fixture body placed too close to the ballast and not much from the ballast itself.

You mean like this?  It's always a good sound, but a dead silent electronic can also be nice, too.  I actually kind of like the sound of 50Hz better.  It has more of an “angry” sort of sound.