The Dimmer probably uses PWM or is a equivalent switched mode power supply, unless it has a pretty large transformer on it. As the name of this circuit topology suggests the current is switched on and off very quickly with different mark:space ratios to give you a varying output current. The switching for these units is typically high in the region of 1kHZ or more, therefore the noise is actually generated from the switching itself, and it is inherent in the design.

I would suggest moving the dimmer further away if possible or trying another type/make.