Thanks to everyone for your replies and guesses as
to what was causing the noise. I found the problem
today. It was a Brother MFC-7150 printer/copier/FAX.
There is an LED that blinks when an ink cartridge is
running low. The noise was cycling with the LED blink.
When the printer cable was disconnected from the printer,
the noise disappeared completely. Apparently the circuit
that drives the blinking LED is a fairly strong & noisy
transmitter at 28 MHz.
While I have had the printer for several years, I usually
unplug it (and all other unnecessary devices) during a
serious contest effort. I forgot to unplug it during my
casual ARRL 10m contest effort, which is when I first
noticed it all over 10-meter CW. . Also, the noise only
occurs when the light blinks when an ink cartridge is
about to run dry.
It's a jungle out there, but sometimes even a blind pig
finds an acorn every now and then...
Happy noise hunting,
RFI mailing list