First thing, try disconnecting your CATV line. If that fixes it, you've got a ground loop problem. A ground loop isolator in the line between the CATV and your system will fix it.

If that isn't it, then try disconnecting random things until it goes away. In particular, if you have different components connected to separate outlets that may be grounded differently, you can also have a problem there.

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Matthew J. Hill
matt@idsi.net