Differance in DC and AC operating and release voltages for an AC Contactor

Because you have some force trying to open the contactor (a spring or gravity), whenever the magnetization approaches zero, the contactor begins to open. With ac, the current goes through zero twice per cycle and, at each zero crossing, the contactor begins to release. This is the hum or chatter you hear. DC does not go through zero and, therefore does not hum.

Now, look at a typical iron core hysteresis curve

http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/HysteresisLoop.htm

you see that some residual magnetization remains when all magnetizing force (proportional to current) is removed.

If the ac current is high enough, there will be enough residual magnetization to hold the contactor closed till the next cycle. Thus, at high voltages, the ac coil does not hum. This is not a good idea usually since the current is often high enough to overheat the coil. A small "shorting ring" can be added to the coil to provide slightly out of phase magnetization which will be enough to hold the contactor during the zero crossing.

In a dc coil, the residual magnetization may be enough that the contactor will not release quickly, and a small air gap is often included in the magnetic circuit to minimize this effect.

So, in general, a dc contactor should not be used on ac and vice-versa.