There was an article in AIRLINERS magazine (in the ask airliners? section.)
"When jet fan blades rotate, they generate a supersonic flow at their tips, which creates and audible shock wave. Because each blade is slightly different (there is a small variation from tip to tip), several different frequencies are created, resulting in the distinct "combination noise." This is the "buzz saw" sound that we hear, especially in the foreward cabin, when the engines are at high power settings such as takeoff roll and initial climb. Some high-bypass engines do not exibit this effect. According to the acoustic engineers at Pratt & Whitney, a possible reason for this is differences in engine inlet and fan blade sizes."
It also says that this happens on alot of aircraft, 747, L-1011, DC-10, 757, 767, etc.
Hope this helps