Civil Air Patrol, the auxiliary arm of the USAF responsible for most SAR missions over the continental US in regards to lost
aircraft. There are exceptions, for example if an aircraft is suspected of being lost
in Lake Michigan it is a USCG search mission and not CAP. DF is the acronym for Direction Finding.
Anyway, the procedure is called an "Aural Null Pattern" . Google
brings up a bunch of links, but essentially you map when/where you start to hear a signal and map where you lose it. Then you can fly a reciprocal course and turn 90 degrees when you have flown half the distance back. If the signal fades quickly you have turned away, so do a 180. Now you can do another leg on that heading and when the signal nulls again, you can chart out the center of both legs (perpendicular to each other) you have flown which is hopefully around where the ELT is located.