ET2013

About 1 hour after parking and shutting down the car in the garage, I was out there again and heard a humming sound (like an electric motor or a stuck relay), which I tracked to the rear left quarter (driver's side), just above and behind the rear wheel well. Venting the gas cap did nothing however turning the key on and off without starting the engine stopped the noise. In doing this the electric fuel pump comes on, which is a different sound than the humming I heard. This had happened once before, about 2 months ago. The dealer suggested today that I simply monitor it until it became more frequent so they would have a better chance to track it down. Anybody heard of this before? What was it? Thanks