2001 Caravan -- Brake noise

We have a 2001 caravan that makes a vacuum noise at idle under the dash every 30-40 seconds. When this happens the brake pedal depresses (falls) slightly and the brake lights go on. There is no apparent
noise under the dash and the hose from the booster to the intake looks fine as well as the fluid level. When the pedal is pressed it feels spongy but the brakes do work. Any ideas would be appreciated.

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