Thanks for the offer Dave, but I think we've got it nailed down. I found an amateur radio site that gave a really good diagnostic procedure for fence charger interference, which has proven to us that it's all the little poor connections we have by gates, etc. They said the first step is to disconnect the fence line from the charger and turn the charger on. If the noise goes away, it can't have anything to do with the charger. Then add a lenght of wire that has no interruptions and secure it with a single known good insulator at the end. Turn on the charger. If there's no interference then the problem has to be with bad connections somewhere in the wire, or with vegetation causing arcing, or something like that. We've found that our primary problem was the crimped copper wire that's used as a jumper onto the steel fence wire. I have to buy a good connector for that joint. We also had a gate connector with some corrosion. It turns out to be really easy with an AM radio to find the source of the problem: just walk along the fence and listen for the hissing and popping as you get close to the short. Thanks for all the suggestions.