Q. Why do some brands of digital servos hum and squeal while others don't? And are the ones making so much noise also pulling more current?
A. By the time they get to us, most modelers have experienced other brands of digital servos (and by and large come to accept the whining and squealing as the price to pay for advanced digital servo technology). And it's also a fact, some servos are noted for emitting quite a loud hum, or squeal - even while sitting at neutral (and more so during operation). Anyway in response to the second part of the question, because of it (the squeal as the servo hunts for position), these servos pull a 'lot' more current than a well tuned and quiet servo like ours (even at idle).
As for why some servos make a lot of noise; it's my opinion this has 100% to do with a lazy designer or a cheap ass company unwilling to expend the effort on making things 'right', most specifically, with respect to tuning the servo's closed control loop (the PID loop). Note PID stands for Proportional, Integral, and Derivative gain and it's how you tune a circuit for the control in terms of the speed with which the motor is operated (basically varying voltage), the distance the servo has to run to get to position and how things change as you get close to the point (the integral part), and the gain or what you do as the position point approaches zero and/or overshoots. The why for all this greatly exceeds the scope of an answer in this format so just accept it as fact.
Frankly, at ProModeler we spend a LOT of time on this and as a direct consequence, our servos are very well regarded for two things, especially. Beyond having a deserved reputation for high quality and outstanding performance, the first of these (and most obvious) is ProModeler servos are significantly more quiet than other digital servos on the market (to the point some aren't sure they're plugged in). And the second thing (low power consumption) being a direct result of the former. Basically, because they're not always hunting for position, ProModeler servos are very thrifty in terms of power consumption (and you'll see folks commenting about this within customer reviews).
The upside of the the former is they're not loud and obnoxious in operation, which seems to get on the nerves of most people (me included). And with respect to the latter, lower power consumption; this means you can use a smaller/lighter battery in your model (for some models because you're not going to use as much current during a flight, aircraft especially, this is a big deal). But it's the squealing like a stuck pig (that incessant hum and whine) that drives most folks nuts.
Anyway, since 'I' don't like it when my servos squeal, I figure my customers won't either and thus, we spend whatever amount of time is required to get it right - sometimes weeks of engineering, which is expensive. This, of course, proves the corollary being, you get what you pay for! And note; there are lots of factors involved with tuning beyond the obvious like how fast you move the joystick and how closely the servo follows (without overshooting) because there's also the effects of the mass of the control linkages and surfaces (and how much play they have), plus details like the natural frequency of the control surfaces themselves (even a carbon fiber surface has a bit of flex). Anyway, this all plays a role in tuning the PID-circuit because it can induce oscillations in the servo if done wrong, or set up that squeal we all hate.
Bottom line? Quite honestly, we find the payoff in terms of time and effort spent on tuning the PID-circuit to achieve a quiet servo to be well worth it. Moreover, while I really don't understand why some companies take a shortcut in this regard, the fact is they do it all the time. And by the way, we see it in all the phases of servo products, e.g. it's not just shortcuts being taken in terms of mechanical design but also in the electronics as you have astutely noted. It's the latter, especially, which is expressed in noise and people object to it a lot.
Of course, this leaves an opening in a crowded market for the discerning consumer. And after all, this is our niche at ProModeler . . . where our tag line pretty much says it all.
Better parts. Better servos. The formula is simple.