Circuit Breaker trips

I recently came home to discover a 15amp breaker that was tripped. The circuit powers a GCFI outlet, 2 vanity style light fixtures and an exhaust fan in the master bath and 3 small light fixtures, the oven hood and a cieling fan in the kitchen. No outlets other than the GCFI outlet are on the circuit and nothing is plugged in. With all of the light switches off, I moved the circuit breaker firmly to off, waited a few seconds, then back to on. The breaker hums for a about a second then trips again.

This is a 15 year old double wide mobile home that I've lived in for about six months. The home was previously owned by the county and operated as an EMS station, so I'm rather confident that prior jobs were done up to code.