Electromagnetic Immunity testing
There are a myriad of things it could be.  Some things to consider: 
Have you double checked grounding/bonding/shielding? Need to know the distance you're testing at, power of received signal at different distances, and characteristics of the signal.  Is it found at other harmonics of 65MHz? Does it coincide with clock signals on the DUT? Seems like if it were the horizontal wires acting as an antenna, then the signal would probably be horizontally polarized. At 65MHZ, vertically-polarized, look for a vertical radiating element at whole-integer fractions (1/2, 1/4, 1/8, etc.) of the wavelength.  Then, look for circuits coupled to it or within it's near-field (wavelength/2pi).