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The two types of interference to AM that I am familiar with is line carried and Wireless (RF).
Line carried can usually be filtered out with a small inductor in series with a capacitor connected from the line input to chassis (L type Filter) Earlier Tube type Auto Radios had a Hash Filter, a simple Pi network with a few turns of wire forming a coil in series with the line and a capacitor on each side of the coil to ground. The purpose of this was to filter out the vibarator points switching. Both the the L and Pi filters work well for line carried noise. Electric iron thermostats (before Wash and Wear) constantly switched off and on to maintain the iron temperature. This made radio listening difficult, so much so plugin power line filter were marketed. The Filters were slightly longer and about the diameter of a 'D" cell, with a male plug on one end a female on the other. Plug one of these in Receptacle and plug the radio into Filter and Mon could listen to the soap operas while she ironed! It is better to eliminate a noise at the source than filter it out. Easier said than done. Sometimes line noise in carried in from the other customers on the same pole transformer you are on.
Wireless interference of the buzz hum and static type is usually beyond our ability to filter because it is a radio frequency noise usually generated by a spark. As it is a Radio Frequency Radiation, the FCC has determined the amount it can radiate. It is a very small amount, if you can hear more than a hundred feet in may be too much. The power company through proper maintenance could eliminate much of this type noise. A cracked insulator, a tree limb against the line or a vine growing onto the line are some of the sources. Other Sources of this type noise although not common are industrial and medical equipment. An AM auto radio tuned below 700 KHZ off station is an easy way to detect Wireless interference. If the noise signal is weak it will become louder as you get closer to it until it gets strong enough to trigger the Radio's Automatic Volume Control. After the avc is triggered, increased signal strength will not make the noise louder.
If someone really wanted to find the source of this type of interference it would require a Standard Broadcast AM Field Intensity Meter. Directional AM Radio Stations would have one. With one of these, the interfering noise can be tuned in, the direction of the source determined and followed until the exact source location is reached.
GF33
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