What is the buzzing noise generated by the engines in this 777 takeoff video?
That buzzing sound is from the tips of the fan blades breaking the sound barrier.  When you're sitting farther back in the airplane, that sound is overpowered by the sound of the exhaust, which is the roaring part of a jet engine's sound.

So in general, sit forward of the wing, you'll hear the fan sound (the buzz), sit aft of the wing and you'll hear the exhaust sound (roar).  It'll vary from airplane to airplane of course, and when thrust is reduced and the fan speed slows, the buzz will subside.