hum from nearby high-tension wires?

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.

Steverw, I have no idea what to tell you. The Electrical contractors in your area must know how to deal with your situation. They should come out to your home for free and give you an estimate on how much it would cost to fix your problem. Ask lots of questions, don't be shy. What kind of electrical earth grounding Electrode system do they recommend? How deep will they need to go? How much $$$$.

Here is a Link to some NECA/IBEW electrical contractors in your area I believe. Did you call any of these? I think you will have the best luck with a Commercial/Industrial Electrical Contractor.
http://www.mbccneca.org/member_list.htm
Are any of them in your area?

You have one hell of alot of power around you. I guarantee you that Substation has earth grounding up the Ying Yang, lightning protection.

What ever you do to solve you problem post back and let us know what you ended up doing.
Jim