The noise is ONLY present with ECS on. It's simply just a spooling type sound reminiscent of an engine start. Just hop into an F-15E that has been ramp started, turn the Radar OFF, and mess with the Bleed Air for ECS.
3-5 seconds, 16-18 sec, 26-29 sec, and as soon as I shut off the bleeds the noise is no longer present.
I turned the Radar OFF as well, but otherwise this is a normal ramp start F-15E. My hearing is good according to my yearly audiograms and I have some good headphones, but I can't be the only one hearing it. Maybe I'm just sensitive to the higher pitch in the noise. I just can't unhear it now. I always notice it and I've been flying it for a while.
ECS Sound Bite Loop.mp4