I had the same thing happen to me twice. The first job was next to a radio station and I tried to resolve it by installing shield cat5 cable with a drain wire. This helped out somewhat but the near field was just unworkable so I had to install an analog switch. If you do have some kinda induction issue you may want to try shielded cable with a drain wire and only tie one end to ground otherwise you may get a poetical difference.
The second time this happened to me I found that two stations were to close to the switch and just added about a 50 foot service loop to each of the close exts and wala worked like a charm no problem since the install.(knock on wood) good luck.

You don’t drown by falling in the water. You drown by staying there.