All electrical devices produce r.f. (radio frequency) signals. Further, all devices designed to receive signals only on a specific frequency will still pick up strong signals on adjacent frequencies (or 'harmonic') ones.
So the source of the interfering signal isn't necessarily any device which is intended to produce r.f. radiation. It could be something like your central heating's timer switch (or that of a neighbour) which is causing the problem.
As Gingejbee suggests, you might need to see if a different channel is available for the doorbell's signal.