Brake booster pump activates every 6 seconds

I have a 2014 prius. Its got 140k miles. The brake pedal has a small amount of freeplay before the car begins to brake. At first that whirring sound which i think is brake booster pump was only heard at startup for about 6 to 8 secongs, and again after shutdown. Now this elect motor / pump is activating for about 1 second every 6 seconds. (In addition to the usual times). And it never stops! Its cycling the whole time Im driving, and also the whole time its off. Im not losing brake fluid, and braking is good, other than that initial 1/8 in or so of travel. Is the brake booster about to fail?