Intermodulation is a source of interference that arises from the combination of two or more signals at different frequencies in some kind of non-linear system.   For example, you could hear strong interference at 14.0 MHz if there are two strong signals at 86.0 and 100.0 MHz that combine in a rectifying circuit, such as an oxidized fence connection or a gutter.

• A Modern Urban Problem
  August 1996 QST, "Intermod"  Includes a homebrew 10 watt notch filter project to remove 152MHz pager interference from 2M handheld transceivers.
• A No-Tune 2-Meter Bandpass Filter
  QST May 2000, pp. 54-55
  A homebrew solution to annoying intermod.

• Intermodulation [Wikipedia]
• Free Software for Broadcast Engineers!
  Intermodulation Products Calculator
• Par Electronics sells intermod filters to combat pager interference