The light in the bathroom is 12v DC, so when you turn it on the fan you here running the power converter, taking the A/C current coming into your unit and sending DC to those lights that are DC.
Jim & Linda Tator
2013 Crusier CF34SS Ford F250 PSD 7.3L
Ride-Rite Airbags Bully Dog
154 nights in 2013 - 4,370 miles
84 nights in 2014 - 3,208 miles