FM Flutter

Flutter can best be described as follows:
^ Repeated pops and hissing bursts heard in the radio speaker during an otherwise good broadcast.
^ Usually this condition exists while traveling in the fringe area of the station and flutter will become more severe beyond approximately 40 km (25 mi) 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 chassis 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 and if the vehicle is parked in one of these dead spots, you will only hear noise from the radio speaker. 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.