With that sort of signal level it must be something really close to you.
First you need to check out your own house is in order, any chance you can use or borrow a battery and run the rig off that and switch off your mains supply and see if the same thing happens, if it clears itself then you know it must be something in your house.
That duration of noise sounds remarkably like an LED PIR light in the area, try keying at night and see if you see it come on.