I might have used the wrong term - but this is the horn you get with idle throttles.

In my case I was at flaps 5 with a descent to final on autothrottle.  Throttles were at idle, horn was singing away.  On the 737 there is a horn cut out for that purpose on the lower right side of the throttle quadrant.  You can see pilots use it quite often in videos  in the same scenario.  (300-400-500).

Joe