After test chamber 6-2, you will enter a between-test chambers section. Halfway through you will join an office section. Follow this to the end as Conly lights the way from the command rail. Near the end of this section, you will go upstairs and find a room full of mannequins. Rather than jumping downstairs at these for 40 seconds or until Conly starts to hum.
The video guide starts at 0:11