I did come across a possible explanation of what you experienced though, of course, I can't vouch for the accuracy since most of it is greek to me...
"Ac coils for relays and soleniods have a shader ring(usually made of copper)on the top of the core. Because of the shader ring, the magnetism developed in part of the core lags somwhat the magnetism of the remainder of the core. That is, there is a slight phase displacement between the magnetism of the part of the core and the remainder of the core. Thus, as the unshaded-core magnetic energy decrease to zero every half-cycle, the magnetic energy still present in the shaded portion of the core holds the armature sealed. By the time the energy in the shaded portion decrease to zero, coil and unshaded-core magnetic energy have begun to increase once again as current increase in value.
Applying AC to a dc coil will cause the coil to hum and if it is a relay it will chatter. The resistance of a DC coil is about 4 times the resistance for an AC coil for the same voltage.
AC coils have inductive reactance in addition to the coil resistance that affects the coil current. The AC current for a 24V AC coil is about double the DC current for a 24V DC coil.
Operating a 24V AC coil on 24V DC could cause the coil to fail as the current would be too great. The current would be very close to 4 times the current for 24V DC coil. "http://forum.allabou...ead.php?t=31610
Edited by DurhamAppliance, 11 February 2014 - 07:43 AM.