What causes microphonics in a tonearm?

Several tonearms i'm considering have been accused of being susceptible for microphonics. The tonearms in question are the Pro-Ject(Oracle Paris-RPM 10.1)Rega(P9) and Rega based(OL) tonearms. Is the poor quality stock tonearm cable responsible or is this a design flaw? As always I appreciate your comments.
Acoustic energy flying around the site of the tt. Put an acoustic screen around the tt, or better yet, move the tt to another room and I think you will see microphonics disappear.
"or better yet, move the tt to another room and I think you will see microphonics disappear."

Try another location in the same room. One more reason your electronics should not be between the speakers. 2/3 of the way down the side wall from the speakers should do it.
I stuck my "used but clean" Linn Basik/Akito on a light but stable table by itself...as the instructions instructed (after initially ignoring said instructions and learning they actually know what they're doing). The resut was as predicted, and has stopped my quest for a better TT right there. Sounds amazing, and it stays until it blows up.
With my current set up I won't be able to isolate the turntable on a dedicated stand. Is vibration from the plinth more responsible for microphonics then the damping capabilities of the tonearm?
Listen to your arm with no sound from the speakers. Almost all arms will have a measure of sound coming from the arm itself, ie acoustic noise generated from the cartridge which is transmitted to the arm tube and then to the air. Sometimes this is known as needle talk. Generally the more compliant the cartridge, the less needle talk there is, which is why a lot of arms are ok with high compliance moving magnets, but have issues with low compliance MCs, (although there are other factors)

If your arm suffers from this, then other aspects such as feedback from the speakers or vibration from the floor via the turntable will exacerbate it.

If the armtube vibrates/resonates then that is the source of the problem. Where the vibration comes from is secondary, though removing it obviously will help, and turntable positioning and mounting is important.