The boxes could be transformers, IF they are low voltage lamps. GU does not say much, that is the type of fitting. the box could also be a connector unit. Are they all the same voltage rating lamps? the buzzing could be the dimmer itself, It may not be rated correctly, depends on who installed them. You need to check all lamps are same voltage and same wattage. You then need to check, if the dimmer is upto the job. After switching off at the mains box, if you carefully unscrew the dimmer, it should say on the back somewhere the rating, some give as wattage some as amperes. IF amps, TOTAL wattage, divided by voltage gives you the amps.
A lot of low voltage units, cannot be dimmed. Depends on the dimmer and transformer/s, they have to be correct. If that fails, get some one in
Sheepdog, March 2011