The hash is coming from Ethiopia. They are jamming The Voice of the Broad Masses of Eritrea
, which broadcasts in the supposedly broadcast-free portion of 40m, starting at 0300 GMT. Their strongest and most consistently-used frequency is 7175, but additional frequencies include 7165, 7130, 7120, 7185, 7195 and probably others. They usually simulcast on 2 or 3 of the above frequencies, and sometimes hop around to dodge the Ethiopian jamming. I have heard them play a cat-and-mouse game by hopping amongst the above frequencies, only to have the jammers follow them within minutes of QSYing. At times I have heard another Ethiopian sounding BC station zero beat the carrier, and then the hash come on top of that, so you have two broadcast stations zero beat with each other plus the noise. The Eritrea station also transmits a different program on 7205, but I have never heard the jamming stations bother them on that frequency, although the jamming has been heard centred on 7220.
The broadcast from Eritrea can be positively identified by listening just a few minutes before sign-on at 0300Z to the interval signal, which is the sound of strumming on a guitar, broken up with announcements in the local language. The sound of the interval signal can be verified at http://www.intervalsignals.net/
(the site is down right now, but they promise to be back up in a few days). Find Eritrea on the list, and click on the link to hear an audio clip of the interval signal.
The jamming usually starts up at about 0400 GMT and continues for 1 1/2 to 2 hours. I suspect it disappears when propagation to that part of the world goes out as they move into daylight (Ethiopia is 3 hours ahead of GMT, so at 0600 - midnight CST - it is 9 AM in Ethiopia.)
I read a report somewhere recently that China had furnished 4 jamming transmitters to Ethiopia. Although the targeted broadcaster occupies only about 10 kHz of spectrum, each white-noise jamming signal is usually more than 20 kHz wide. When all 4 jammers are running simultaneously, they pretty well render the entire supposedly broadcast-free segment of 40m useless in N America.
I first noticed this noise right after the broadcast stations first vacated 7.1-7.2, but it had probably already been there for a long time, masked by stronger broadcast signals coming in from Europe.http://www.iaru-r1.org/index.php?option=com_content&view=article&id=308:radio-ethiopia-which-is-jamming-voice-of-broad-masses-from-eritrea&catid=39:iarums&Itemid=87