FM Flutter

Flutter can best be described as repeated pops and hissing bursts heard in the radio speakers during an otherwise good broadcast.
^ This condition usually exists while traveling in the fringe area of the station.
^ Flutter will become more severe beyond an approximate 40 km (25 miles) of the station.
^ The signal loss becomes greater away from the station, until finally noise takes over and reception becomes impossible.
^ Flutter may also be noticed near the station because of the line of-sight characteristics of FM radio waves.
^ This condition can happen when a building or large structure is between the radio receiver and the station being received.
^ Some of the FM signal bends around the building, but certain spots have almost no signal.
^ Some of these signal losses are only a few inches wide. If the vehicle is parked in one of these dead spots, you will only hear noise from the radio chassis.
^ After moving out of the shadow of the structure, the station will return to normal.
^ Flutter will not occur on AM because the AM radio waves are much longer than FM waves.