I have a 1999 KIA Sportage, and its doing the SAME thing. Its a short somewhere that is causing it. If you disconnect your positive in the battery, get a can of electric parts cleaner, and take the relays out. You want to spray the inlet for the relay, take off the clips on the bottom of the whole box and also spray in there.Do not be shy with the cleaner, theres alot of dirt and dust. Let that sit and dry for atleast a few minutes, as it IS flammable. After I did that, I plugged the relay in and no buzzing for alittle while.
If that doesnt fix it than you must locate the short. Im not a mechanic but this is what ive tried, and it worked temporarily. The next day I woke up to it buzzing, I remove the relay when I turn the car off and plug it back in when ready to drive. Its not your fuel pump, or your crankshaft sensor, its a loose, cracked or messed wire somewhere. If you know somebody that is good with electrical its probably a very easy fix. this just happened to me a few days ago. Once the car is started it doesnt buzz. Although I drove it to advance auto and had the computer connected to see the engine codes, and there were cylinder misfires, fuel pump and crankshaft sensor light. the KIA was also sputtering and almost stalled. After I cleaned it that stopped.
In a few days im going to disconnect that whole box and see if one of my relay wires is loose.
Source: My own trial and error.