I have a cheapo 400 watt ballast from HTG and it is nearly silent. It buzzes lightly, , but I cannot hear it with the tent door closed. I have another 400 watt ballast that I picked up used from a construction site (they were changing out all the lighting and were throwing a bunch away) that is almost silent as well.
Not saying that they will not eventually start to make noise, but the magnetic ballasts I have worked with are not that noisy. Of course, the digital will never make any noise and it is also a bit more efficient. Comes with an upfront price that is higher though.