UV
This may be in the exams but I am 99.9% sure it isn't true. The signal emitted from a VOR is continuous in both time and the radial information and does nothing special at the cardinal points.
All sorts of interference scenarios are possible; one cannot rule out anything outright really. Most planes flying are very old and their wiring is similarly old. Where cables are shielded, the shield connection can be corroded or not even made in the first place - most avionics shops are no more than followers of wiring instructions; they usually aren't electronics experts. Look at how many planes have their radio transmissions badly affected by their strobe inverters, and that's just a really simple case.
A multiple of RPM is perfectly possible if the magneto/ignition leads are radiating.