Engine noises on take-off
Might seem like a silly question, but on take-off, the engine noises on some jets, like A330 or B757 are very loud and you can alsmost hear a "vibration" in the noises that can be heard.
This noise soon calms down and fades away after about 10 to 12 minutes, even though the aircraft is still climbing.
Can someone tell me what is this noise and why does it just gradually fade away - almost without noticing?