Most AC-DC converters are switching power supplies that switch on and off at very frequency (ie. 10kHz). When you have 2 switching supplies close together they

Most AC-DC converters are switching power supplies that switch on and off at very frequency (ie. 10kHz). When you have 2 switching supplies close together they can introduce some interesting harmonics, especially if they aren't shielded well. Likely, he switched out incadescents for some other lighting type and if he was using flourescents you can definitely hear it.