Random interference problem with Shure UR4D
I would need to hear it to be completely sure (does it sound a little bit like slow morse code?), but it sounds like what you're hearing is probably cell phone interference. Certain cell phones are more likely to cause it (anything with PTT capability) because of their transmitter power, but any cell phone close enough to the receiver could give you grief. If you're playing in unpredictable places, try to get the antennae well overhead of the audience, with a straight shot to the transmitters, and as close as is reasonably possible.