Human hearing is fantastically sensitive in dynamic range, and able to precisely discriminate different frequency ranges.
The Zvezda module's integrated thrusters are sometimes used for reboost; this is a pair of S5.79 rocket engines with 3kN thrust each. I can't find a video of such an engine in action, but here's a test of a Chinese engine of similar size and, for comparison, a test of the much smaller (400N) SpaceX Draco. The alternative to using Zvezda’s thrusters is to use thrusters on a Progress resupply ship. Those thrusters are of generally similar scale.
You can hear that the sound of the engine has a lot of high-frequency hiss. That's sound transmitted through the air, of course, and the frequency spectrum of sound conducted from the engine to the body of the space station would likely be rather different, but I imagine that a fair amount of hiss would still be audible inside the station, which might be a contrast to the lower-frequency "rumble" of the station's air system.
I can't hear a difference on the video between the during-burn and post-burn soundscape, but that's sound going into a cheap mic, encoded to low-bandwidth video, and reproduced on cheap laptop speakers, so I'm not surprised. I imagine that after being on the ISS for a while, you learn to tune out the steady-state ventilation sounds, and the sound of the boost engines themselves stands out as distinct and unusual.