These are the commercial jet engines you see, like the Boeing 777, 747, Airbus A330 and A380s. Though there is a considerable reduction of the engine thrust, at these altitudes the lower drag associated with low air density means a more economical flight with a higher True Air Speed.
Crepitus in the knees may be caused by knee injuries such as meniscus tears. These are fairly common in people who play sports, jog, or run. A meniscus tear can cause crepitus as the joint moves. Chondromalacia patella is a dull ache behind the kneecap, usually caused by overuse or injury. There will be damage to the undersurface cartilage covering the kneecap.
The name of this noise is Crepitus. Crepitus is a medical term to describe the grinding, crackling or popping sounds and sensations experienced in the joints. It is possible that the noise is coming from scar tissue which is normal wear and tear on the joints. This sound happens when two rough surfaces in the human body come into contact.