Jfrech:
When I tap on the trough, I hear a *sound* but a sound is not necessarily a resonance. The sound that I hear when I tap on the trough quickly dissipates. Now, you can impart a lot more energy upon a tonearm part with your finger (or a screwdriver) than can ambient sound energy from your speakers. So, what that means is, when resonance is lacking, sound energy has no chance of having any impact on the sonic performance of the tonearm.
Resonances are something different. What is different about a resonance, and why resonances are such a problem, is that when a resonance exists, sound energy is trapped within the object, which then becomes a source for additional sound propagation. In effect, sound energy that enters the object tends to stick around for a lot longer. That's why the tone that you hear from a tuning fork hangs around so long - it's because of the resonance at the tuned frequency of the tuning fork. On the other hand, other frequencies tend to get wiped out rather quickly, so that's why you don't hear them.
I can't, and don't, comment on what people think they are hearing. For example, I will not tell you that you aren't hearing what you say you are hearing. I simply lack sufficient information to make such a statement. The question that I ask myself is: "is there an explanation that is based in sound physics or electronics reasoning that explains what people say they are observing?"