2D
I'm not convinced that an engine or accessories could generate significant harmonics in the VHF range, excluding of course spark noise from faulty installation which would affect the entire VHF spectrum.
In theory electronic equipment like FADEC or electric RPM generators might produce VHF interference, but that's all a bit specialised.
It is true that the alternator can generate hash and you can demonstrate this by tuning your car radio to an unused FM channel. You are likely to hear an engine speed dependant whine.
However, I think it is reasonable to assume that aviation alternators are designed for RF compatibility and do not radiate significantly. The whine on the car radio is likely to be coupled by the power wires, rather than by RF, and may enter the audio section of the radio rather than the front end.
So an alternative explanation might be that it would be fairly easy for the alternator to 'modulate' the aircraft battery DC at some multiple of engine speed. This DC ripple might be able to find it's way through the Navaid power supply and on to the VOR needle. A simple test would be to trip the 'Alt' switch.
David