cttypes

I have a 2003 TL type S and recently noticed that when I turn on the parking lights and eventually headlights on, there is a loud humming noise inside the cab towards the back. The only thing that comes to mind is that the rear brake light is making a noise because it's been disconnected (by the dealer I guess before I bought it) when they hooked up the rear spoiler brake light. All the lights seem to be working with no problem though. Anybody experience anything similar? It's not that horrible to deal with, I just hope it's not the beginning of something worse....thanks.