Help: Buzzing Electrical Noise

Would anyone know what does a buzzing noise (sounds like the one you would hear from a a high-voltage electrical line) means whenever I start my car (a 2001 330ci)? I traced where the sound is coming from, and it seems to be coming from under or near the gas tank and/or near the rear axles. Recently, the "Service Engine Soon" light has been coming on and would stay for several days before disappearing again. I don't know if this has to do with the buzzing sound.
Any insight to this is greatly appreciated!

Have the dealer correct the Check Engine light.

Ask him to determine the cause of the buzzing noise coming from the location of the fuel pump.

Later,

Most likely the buzzing noise IS the tank-mounted fuel pump. It's not something you notice unless you happen to be around the rear of the car when the engine is running.

take it to autozone part stores they check the engine light for free. and tell you what code is coming up