1998 Ford F-150 running water sound

I recently had my oil changed and radiator flushed. There was some dripping fluid, on my driveway that appeared to be antifreeze a few times after the engine was flushed. When I accelerate, I hear the sound of running water coming from inside the dashboard. It's very distinct. The heater core has been replaced twice over the last two years (every December!). Help please.