Buzzing sound is fuel pump
The sound you hear is your fuel pump mainly the check valve ball rattling in it's housing. Not to worry unless the sound stops, where by the pump has stopped pumping as well. If you turn your ignition key to position 2 without starting, you'll probably start hearing that sound for a few seconds before the line pressurizes.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan