How much flourescent ballast humming is too much indicating replacement?

I have some ballasts that don't hum at all, and some that hum loud enough
to hear them ten feet away and some that barely hum, but they do hum.

For the ones that hum noticeably (about the sound of a refrigerator
compressor), I know that they need to be replaced and I know that replacing
the $20 ballast is probably substandard to going all LED so for the loud
hummers I will go to the expense of replacing the entire fixtures with
LEDs.

But LEDs will drop in price by a lot over the next few years so I don't
want to replace any ballast or fixture that doesn't need to be replaced
right now.

So my question is all about the ones that hum ever so little?

The hum is about the level of a soft whisper. You have to stop to look to
notice, otherwise you wouldn't notice the minor subtle hum.

Is that soft-whisper ballast telling me to replace it also?