[bragr]

Basically they were just trying to track down weird noises on the recording. They also went and recorded audio on a similar UPS MD-11 to try to identify the source. According to the NTSB:

"This high pitch ringing sound, primarily noted at approximately 6.35 kHz, occurred at 17:13:05.5 EST, shortly after the aircraft rotated for takeoff, and continued with varying amplitude throughout the remainder of the recording. Additionally, a tone at about 2.1 kHz was present along with the ringing that could not be identified."