We have a local telephone company complaining to our utility that they have noise on their system. We have investigated and found a fairly significant 9th harmonic on the power line. This seems to play havoc with phone systems. What is a likely cause of a 9th harmonic? We are a rural utility with little industrial load. The area in question, however, has a lot of pipelines and cathodic protection. I understand there are filters readily available. Where are they to be found and how much are they likely to cost?