buzzing sound

on replacing one-way with a dimmer i not only have a buzz from the switch (normal) but also from the light itself (noisy!!). the light is halogen GU4 low pressure 12v 10w x 10bulbs. THe dimmer said 400w max (halogen 250w max) i believe i only have 100w on it (10w x 10bulbs) any ideas as it is impossible to have light on due to noise!!
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depends which way you want to look at it for your answer. either the dimmer switch is wrong or the lights are wrong.

you can not use an ordainairy dimmer switch on low voltage lights.(that is the dimmer you have now)

you will either have to change the dimmer or the transformer (which is why you say the light is buzzing, its the transformer for the lights)

or you could change the lights transformer, to an electronic dimable transformer.