DIAGNOSIS That's the unit's "I'm not happy with my seating" beep. The most common cause is the cooking pot not being fully and correctly seated inside the base.
FIX Take the cooking pot completely out. Look inside the base and make sure there's no food debris or oil on the silver heating element ring or the central temperature sensor in the middle. Wipe it clean with a dry cloth. Now, place the pot back in, but don't just drop it. Lower it straight down and give it a firm clockwise twist until you feel it click into place. You should hear a distinct click, not just a slide. The handles on the pot should align with the handles on the base.
IF THAT DIDN'T WORK Unplug the unit for a full five minutes. This is a hard reset for the internal control board, which can sometimes get stuck in a weird state after a power flicker or previous error.
HEADS UP Never, ever add oil or cooking spray directly into the empty base where the heating element is. That's a fire hazard and will gunk up the sensor. Always spray your food or the basket/tray.