I don't understand how the frequency goes up with more current. It's a fixed off time of 30us, so the only thing you can do to get more current is increase the on time to get a higher mark space ratio. That means the total period must be longer, so the frequency lower.
One possibility is the switching is ultrasonic but the two haves of the chip beat with each other and produce a sonic frequency equal to the difference.
You could try shorting ROSC on the A4988 to see if it then as loud as the A4982.
One possibility is the switching is ultrasonic but the two haves of the chip beat with each other and produce a sonic frequency equal to the difference.
You could try shorting ROSC on the A4988 to see if it then as loud as the A4982.