13560 + and - several kHz is one of several industrial frequencies used for heating various materials in factories. There is a large industrial/commercial area in Elk Grove Village IL just west of O'Hare field that is about 5 to 6 miles south of me. 24/7 there are various rhythmic weak carriers coming from that area. Another freq is twice that at 27120 kHz which can mess up the CB radio band. There is, or was, a factory in Wheeling IL about 5 miles north of me that used that freq. I understand that there is also 6780 kHz (half 13560) but I never heard anything there. There might also be 40680 (13560 X 3).