Multi path -  by Eddie Runner (NU5K) 
Sometimes FM radio sounds scratchy and bad because of multipath interference. Multipath (often called picket fencing or flutter) happens when FM signals bounce around between the buildings in a city, or other large obstructions, this bounce causes a reflection and your FM radio tries to lock onto the original signal as well as the reflection! This phenomenon occurs mostly near the buildings and worse between the buildings, but sometimes even fairly far away from the buildings. The static is more noticeable when moving but it is possible to stop your car in a path, if you do stop in a path you can usually ease your car forward and the reception will change, this is a telltale sign that it is indeed multipath. You can also drive down the road till you hear a flutter and then go around the block and come back to the exact same spot and the flutter will be exactly the same and exactly the same spot. 

To identify multipath first try the AM radio and make sure the AM is strong, if the antenna has any problems they will usually cause the AM to be terrible or weak. Assuming the AM radio sounds fine then the antenna is good. Next step is to tune the FM radio and see if it sounds normal, typically multipath wont be very noticeable while sitting still, if the stations sound very bad be sure to try many stations down the dial. If all stations sound bad, then of course the receiver may have a problem (might be broke!). If the stations sound fine or some bad and some good then the next step is to drive the car through some areas where the static occurs and then make the block and go past the same spot with the same station and verify it always occurs at that spot. This will verify there are places you can drive where that radio station sounds fine, and places where that station sounds bad.
THIS IS MULTIPATH!!

To get rid of multipath static may not be easy, most folks would rather live with it than go to a lot of trouble to get rid of it. For those that cant stand it though the secret is to reduce the sensitivity of the set so that it no longer tries to lock on the reflected signals, only the main signal. Reducing the sensitivity might be as simple as pushing your antenna down a ways.  Some factory car stereos reduce the multipath by automatically reducing the receiver to MONO and some even have multiple antennas (usually in the windshield)…

Multipath I have found over the years bothers folks very badly because they think their receiver is busted, but once multipath is explained and they realize there are only a few spots in the average city where the flutter occurs as they drive, they get to know the bad spots they drive through and feel comfortable in that their radio isn’t really busted.