<p>I've upgraded my stepper drivers.</p>

<p>I'm looking to understand why my stepper motors made noise in the first place.</p>

To make a stepper perform a step, block signals are send to energize the coils to position the rotor. Such a block signal causes abrupt motion and triggers harmonic frequencies. This is audible as stepper noise. If the block signal is smoothed, the motion is more fluent and less noise will be observed. A similar effect is achieved using micro-stepping.
It could be that the new stepper drivers use more/less microsteps
1)
or a smoothed/block
1)
signal opposed to the previous drivers, hence less/more
1)
noise.
1)
The question does not state if the noise is reduced or increased, but noise reduction is most probable