[RFI] Burst of buzzy tones from speaker of turned off radio

[Barry Merrill, W5GN]

We were staying in a motel off the Interstate in Blytheville, AR,
and the AM Clock Radio, which had been left on but had the volume
turned all the way down,  had several instances (5 times in 3 hours)
when a burst of cw-like, raspy tones, sounding like three letter 
U's run together, were heard quite loud.

I've also heard this kind of UUU buzzy tones occasionally in my
car radio, and in an audio system at a bar.

Are these bursts coming from a cellphone nearby, or does anyone
else have an explanation? 

73

Barry, W5GN