I googled around some old R/C sites and didn't find anything promising. Only specific opinion was from someone that sounded as old as you and I are.
The servos we use in our planes are notoriously noisy. The digitals can be less noisy, but they also have a "whine", part of the digital technology, and it can be aggravating, as well. And, the faster they are moving, the more torque they are generating, thus, more noise. You are right, the mounting can help quite a bit. You can also cut a foam block, that can fit around it, and that will help quiet them down a bit. All the servos have a 3 or 4 gear train, and that's where the noise comes from, plus, add the whine from the digital circuitry, and the digitals can be louder over all. Most of the cheaper ones have nylon gears, and the more expensive ones are metal.
The big thing is, the servos are used for moving flight surfaces in flight, where the noise doesn't matter, so there are no requirements to be quiet. So, NO one makes a "quiet" servo. Other than padding and mounting, or going through a bunch of servos, there's no way to get a quiet one.
I have actually saved a plane stuck in the trees, because of the noise - we lost it, had NO idea where it had come down, but by working the control surfaces, we finally heard the servos, and found the plane...
Been flying RC planes for 25 years...ain't heard a "quiet" one yet! Again, you can pad it quite well, and there's really no need to worry about heat build-up - the case is sealed anyway, so there's not much need for heat dissipation.