They're normally switched. I can hear mine (Hobbico Accuglow) make a buzzing noise.
Try this:
http://www.uoguelph.ca/~antoon/gadgets/glow.htm
BTW, the resistance of a glow plug gets higher as it gets hot. May have an impact...
The new Hobbico Accuglow II also has a charger you can clip your C Cell glow ignitor on to. Very convienient. Hard to beat the price with a home brew setup.