It’s a well known among IT experts method of reducing fan and disks vibration. Electrolyte in PSU’s smoothing capacitors crystalizes and the capacitor starts exhibiting a dip in conductance at specific frequencies. In general it’s not a problem and, where it is, manufacturers employ
spectrum spreading (you may recall this advertised on your old mobos). But it remains a problem in devices which rotate at fixed speeds, like computer fans and HDDs. Disks are particularly vulnerable, because their high inertia keeps the spinning frequency constant. Hitting the computer case in the right way produces enough of higher harmonics to make crystals in capacitors to break down, restoring unrestricted flow of energy.
Dave is wrong on one thing: you can deliver MHz range vibrations. This is what
medical ultrasonography uses. The reason it doesn’t work for reducing spikes is the wavelength. Sound wave at 10 Mhz has wavelength of 1 mil, that is less than a thickness of US dollar bill. The wave passes through the capacitors and inductors as if they weren’t there, instead of being absorbed.