A bass tone is coming out of the toilet
Sometimes this is a resonant refill valve -- pushing the toilet, or a minor leak through the flush valve, shifts the water level just enough to open the valve a hair, and then it bounces between open and closed a few times until the water level comes up enough to close it more reliably. Fiddling with the water level may help, but replacing the refill mechanism is likely to help more. 

(And since this is telling you that the flap valve is leaking a bit, you might want to consider replacing that too while you're at it.)