No, it really sounds like a server room down there. You don't hear much of the APU, which is at the very back of the aircraft, ~60m away, with the exhaust pointed away.
I’m almost certain it’s the APU. Notice how the sound and volume hardly change when he opens the door from the server room into the cargo area. 60m away isn’t all that far for a big turbine engine. Those things are deafening at idle speed.