weird humming sound...

i've noticed the past few days that i have a weird humming sound coming from the rear of my car after i shut it off. it sounds like it is coming from the back seat/rear wheel area. maybe it has something to do with the fact that we've been in the 90s the past few days, but it's just really strange. it's not a buzzing...but kind of like a small electric motor hum. if anyone wishes to make a guess of what it could possibly be, i'd appreciate it!

[northernkanuck]

could it possibly be your antenna motor??

[svx_commuter]

Fuel Pump under the back seat. (There aren't many motors back there. (2)) Does it change with fuel level? Have you run out of gas often? It shortens the life of the pump.