Topband: NOISE PROBLEM

[Mr. and Mrs. Magoo]

This AM I was fortunate to work KH2/N2NL but it was touch and go as I am 
being plagued by an intermittent noise which typically takes a 559 signal 
right into the noise.....this AM as the DX was giving my report.  It begins 
and slowly builds up, peaking after about 10 seconds and then lasts for 
about 30 seconds before suddenly cutting off.  A couple of minutes later it 
repeats.  It is directional, right off the end of my 330 degree Beverage. 
Other Beverages at 10 and 280 degrees do not appear to experience this noise 
problem.  The sequence continues for about 30 minutes, beginning around 
1130Z and then is gone.

Has anyone experienced anything like this before?  Can anyone suggest a 
cause so I can start hunting down the source?

Bill VE3CSK/VE3NH