Com Analyzers
You might try checking with the local Amateur Radio club to see if they or one of their members own a full blown communications analyzer. Hams love a good "fox hunt" and you might be pleasantly surprised at how willing they are to help you out...just like most modelers.
You might also want to check for "broad band" noise generated by any faulty power line insulators, etc. that may be close by. Using the analyzer, you should be able to track the source to a span or individual pole.
Just some thoughts.
Jim