Yes, it sure can cause RFI, but it will generally be relatively short
lived and periodic.

If the thermostat in the pad is a simple bimetallic strip and doesn't
have significant "snap action" the contacts will tend to have lesser
contact as the temperature rises to the set point. This lighter contact
can cause less than perfect electical contact between the contacts,
creating RFI that is quite severe, until the contacts actually open.
This can last several seconds, depending on the thermostat. A similar,
but less significant problem can happen as the contacts close.

I get a very similar high level RFI noise just before the water pump on
the house I'm renting is about to switch in. The noise begins at a low
level, then increases to maximum over about a half second, and continues
for three to ten seconds, until the pump finally starts. Once the
contacts make a good enough circuit to start the motor, the noise goes
away. A similar noise happens with about a half second total time when
the pump is about to stop. Changing the set point of the pump causes
the time to vary some, and replacing the pressure switch worked for
about six months before the noise returned. Happily, we should be
moving into our new home in a couple of months, so can leave this pump
behind.

As already noted by another poster, filtering can help a fair bit, but
may not totally eliminate the problem.

On the other hand, if you are hearing longer term buzzing type of RFI
particularly of a broadband nature, suspect any dimmer switches or
"touch" lamps as the potential culprit...

--Rick AH7H

Dave Platt wrote:
In article c49Dg.368$Ji1.130@trnddc05,
Amerigo Vespucci wrote:


Would a heating pad cause RFI ? My wife has intermittent back problems and
when she uses her heating pad it seems I get spikes of RFI on my AM
reciever.


That's distinctly possible. Almost any load on the powerline can
create RFI (either radiated or conducted) if its load-switching
circuits cause sudden current changes on the line.

It's possible to build AC power-switching circuits which minimize
current transients and thus minimize radiated EMI. If I recall
correctly, good ones will switch on when the AC voltage crosses
through zero, and switch off when the AC current passes through zero.

Your wife's heating-pad controller or thermostat may very well have a
cheaper switching circuit which pays no attention to the powerline
phase. If so, it'd probably generate a short spike of noise whenever
the heating current was turned on or off.

You could try plugging the pad into a good AC power-line filter,
and/or using a snap-on ferrite or two on its power cord (wrap the
ferrites in soft flannel :-)