I have a Chevy Silverado 99 V-8 FWD 1500 with 60k.
Has a noise like gurgling or rattling coming from under the dash in the center. Comes on at exactly 1700 rpm no matter what load, gear, idle, ac on/off, heater on/off, fan on/off. Stays at all rpms above 1700.
Can't hear it outside with the hood up. Only inside with doors/windows closed.