Ah, so it is happening when the system is putting the motor into inefficient mode, to create excess heat to warm the battery. In that case it'll be running more current that can increase sub-audible sounds to the point you can hear them (they overcome the noise-cancelation stuff Tesla has designed, for example), so that makes a lot of sense.
Another possibility is the noise being different is because the running the coolant is being looped differently. There's this 4-way valve that routes the coolant dependent on where the system is trying to move heat from and too.