Are you using an indoor antenna? Could be generated by some electronics nearby. The 6m band has a lot of unlicensed devices transmitting, such as wireless thermometers, microphones, thermostats, etc. These are low power (supposed to be low power) transmissions usually less than 10mw but even at low power levels noise can propagate for quite a distance. 6m is secondary for amateur radio so it could even be a licensed commercial service using higher output power.
I haven't heard much on 6m so far this year other than a couple local nets at 50.2MHz. Hope we can get some propagation this spring... if it looks promising I'll have to put my 6 element yagi up again....