Ballast fluorescent causes circuit breaker to buzz

If it's correcting it correctly, there is no out of phase current, and it would behave like an incandescent lightbulb, which it doesn't. Incandescents don't make it buzz.

Indeed - I've never heard a capacitor buzzing like that. I assumed the buzzing was coming from the ballast, but that's in series with the removed tube so doing nothing. I looked inside it and I'm pretty sure it's simply a cap. Nothing else.

I wonder what it was doing to the supply line to cause the buzzing? Some weird kind of oscillation?

I didn't realise you had it going with the tube lit. My new theory is that the capacitor was sparking over internally, creating great spikes of current at 100Hz. Maybe a slight blue glow would be seen through the plastic case in the dark. It would be interesting to measure the current in it.

It buzzed equally with the tube lit and the tube removed.

Unfortunately I put it in the bin, which has now been emptied.