I have a similar noise under the left side of the dash maybe behind the glovebox where the fuse board is located. I notice it starts when the key is at ignition the 'on' position. Also noticed that it will stop when the radio is switched off and key turned to off. Could be a relay on the board but have yet to investigate as everything works ok.
Mine makes that same noise. I wasn't sure what it was at first, but then started noticing it when I would turn the fan of and then on again. It's louder or softer depending on the weather, I've noticed.