Its not really a hum, but it does sound like what a jammer would do (although I don't think its a jammer, but a bad feed or something). I hear the same thing on several of their frequencies.
Was listening to them today on Twente's SDR. The program came on & was strong. With about a 1/4 - 1/2 second delay, could hear the same thing, but much, much weaker (usually could only hear the delayed piece if there was a break in the first one's audio).