It's called "passive intermodulation"... Read this excellent writeup by Narda Corp (a manufacturer of high quality microwave stuff): <http://www.nardamicrowave.com/east/appnotesrefregpdf/WIRE0048.pdf>
Look for the section titled "CAUSES OF IMD IN FILTERS AND FERRITES" and points 1 and 6 are the most applicable. Point 3 is why Stationmasters with internal cracked elements create duplex noise and IMD. Mike WA6ILQ
