Try just turning your key to the "ON" position... in a lot of the E36s the fuel pump on one of the few things that is energized in the "ON" key position. If you get that same sound.. then its probably your fuel pump... after a while they get loud... then after another while.. they will die.. I've had one die before... not cool.. same your self some time and money.. and go a head and buy a new one and go to pelicanparts.com and look for the DIY on replacing the fuel pump...
Remember.. make sure you get someone else out there to give you a second ear towards trying to find out what is wrong... this is just a guess to what might be wrong..
Again... Good Luck
1997 E36 M3 1990 E30 325i