Welcome to the world of BMW! A '97 740i with only 45,000 miles has a long, long life ahead if it's well maintained.
Sort of a wild hunch but you might check the auxiliary fan in FRONT of the radiator. It's supposed to run anytime the A/C is on (and perhaps at other times as well depending on the coolant temperator). Get out of the car and walk to the front, bend down and look inside to see if the aux. fan is the source of the noise. If so, I wouldn't worry about it.
This is a good forum for help. Another good one is at
www.roadfly.org.