I suspect that the noise is not actually coming from that capacitor. Yes, some ceramic capacitors can exhibit enough of a piezoelectric effect that they can make audible noise under the right circumstances. However, this pretty much has to be helped along by being mechanically coupled to something that acts like a sounding board, usually the PCB. You said "disk" capacitor, which implies thru hole. The size fluctuations of such a disk capacitor would couple to the PCB quite poorly, and the disk itself is quite small and can't radiate a lot of sound.
Much more likely, there is a transformer in this battery charger, and that is what is vibrating. Each little section of wire experiences a sideways force due to the current thru it and the magnetic field around it. It's quite common for transformers to whine for that reason. Magnetorestriction can also occur, but usually plain old magnetic forces outweight that considerably.
This is apparently a line-connected high-voltage circuit. Anyone that has to ask here shouldn't be messing with such things. Even if you don't electrocute yourself, you could be making the device unsafe so that is electrocutes you or someone else later, catches fire and burns down your house, etc. The simplest solution is to get a different charger. Well designed chargers have audible whine taken into account as part of their design. Something slapped together for the lowest possible cost for the mass market from some untracable factory will be more prone to whining.
Another possibility is to put the charger inside a cardboard box. However, be careful that it does not overheat. If the box is several times the size of the charger, then enough heat can still probably be dissipated. Of course that's going to make it bulky. Unfortunately, most things that dampen sound will also thermally insulate. Again, getting a better charger is a better answer.