If the interference happens low to the ground, like on final approach and maybe 15 feet high or lower and the interference always happens in the very same spot, it could be coming from the ground.
Oil companies actually can find oil by detecting magnetic anomalies. I have flown at two sites that have these anomalies present. They may seem to be seasonal and they may diminish for several years and then re-appear.
The real signature of this type of interference is the airplane is hit momentarily at low altitudes and over the very same spot. If you are experiencing interference at higher altitudes, then this is not the source.

Here, in Houston, one of the clubs was experiencing interference . It was tracked to a radio station's antenna. They claimed that they had no equipment capable of omitting a signal on RC frequencies. But it turned out that improperly ground equipment was causing the tower to transmit on frequencies other than it was assigned.