hum from nearby high-tension wires?

Turned out to be a loose connection.

There was no radio noise, even right under the wires.

But now I know a lot more about grounding, thanks for the advice.

Two things about so-called 'harmful' EMF - 1) The fields decrease at a square of the distance, i.e. if you double the distance away from the line, the field intensity goes down by 1/4, triple the distance 1/9, etc. So while there is detectable electromagnetic radiation below the lines, its pretty weak at ground level. 2) If EMF from power lines causes cancer,etc., why isn't there any epidemic among power company Lineman (there isn't). And no, I don't work for an electric utility. But there is No solid proof of EMF from power lines being harmful (unless, of course, you come in contact with one and are grounded)...

Steverw, have you called someone out yet to check your grounding?

What part of the country do you live?

With a plastic main incoming water line you should have a minimum of two 8ft ground rods. One wire coming from your main electrical panel to one of the grd rods and continuing to the next.
Minimum of 6ft apart, 8 to 10ft apart is better. Three or more ground rods is better.

If you have not called someone to come out yet, when you do have the electrician verify just how many grd rods you have. Unearthing the tops of the rods, cleaning and reterminating them as needed.

Depending on what part of the country you live, soil conditions is very important for the type of earth grounding electrode system you have. And depending how old your home is the electrician will be able to advise you what is now required by local codes for your area.

Also while the electrician is there have him check and tighten all termination in the main electrical panel.
Jim

Jea48: I live in Santa Cruz, CA. I've called some local electricians, but so far I haven't invited anyone out yet because I want to research a little bit more.

Specifically, our house is on a hill that is a big rock. Dig down 8" and you hit this yellowish rock. This area used to be quarried for limestone. I think the type of geology is called "karst". The rock is not hard like granite, but it is hard enough that you need to chip at it with a pick to make any sort of hole.

The thing I'm wondering about is whether this rock (is it limestone?) has good grounding qualities. The local electricians I have called say they don't know and don't have the measurement tools to test, all they do is visually check compliance with local codes. Do you know if there is a lab somewhere I can send the rock for testing?

In my own visual inspection, I can see that we have just one ground rod, which appears to be just some rebar pounded into the ground; I don't know how deep it goes. The earthing strap is all corroded. So this definitely needs some attention!

If you can't drive a rod down into earth, another approach is a metalic mat, say 4X8 feet, which you bury as deep as posible.