The hush house was always fun when running engines that had AB blowouts or compressor stalls. A big fireball coming out of the intake for a split second before being sucked back in.
The sound at 6:18 & 6:22 and during the video is made by a device called a CENC motor. (Convergent exhaust nozzle control) This motor opens and closes the nozzle and the sound is the air coming ouf of its (the CENC's) exhaust on the bottom of the fuselage.
Westover ARB, Mass. C-5. Awesome aircraft to work and fly on.