Of course you have surge suppressors, Sorcha. But how old are they? Haven't I read that they wear out after a few years and need replacing?
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We used to have a gremlin at our house. When you turned the rheostat on the dining light up and down, the speakers in the living room hummed louder or more quietly in sync.

The problem was this: down in the basement, the wire to the rheostat crossed the speaker wire going to the kitchen speaker at a right angle. I believe I remember from physics classes that a current in a wire generates a magnetic field. The increasing current in the wire to the dining room light created a magnetic field. This magnetic field then induced a current in the speaker wire that it crossed, and the speakers responded to the current by humming.

I learned from a friend who wires McMansions that the thing to do is to make as many wires run parallel to each other as possible. Likewise, we fixed our problem by moving wires so they did not cross each other.

This may not be your problem, but it's interesting to know.