unowen wrote:As for the audible buzzing at the switch, do you think that's a defective switch and may be adding to the problem, or do you think it's an effect of the existing line noise?
X10 brand switches tend to buzz worse than other switches. Even the same identical switch model can buzz differently from switch to switch. I read on comp.home.automation that the power cutoff slide switch is manufactured poorly, which contributes to the worse buzz on these switches.
That said, just about all of the dimmer switches will buzz a little when the light is not at 100%. The filament in the bulb itself can also buzz when not at 100% (it is worst at 50%) -- more heavy duty bulbs can help dampen the noise. I've never heard of this buzz causing X10 interference on the powerline, however.
Are you hearing the buzz at 100% brightness on that switch? It sounds like you still heard the buzz on the phone line even when you had the power off to all circuits, including the light that had the bad buzz, so as you mentioned it doesn't sound like the dimmer itself is the cause of the noise.
regards,
matt