My toughbook does this as well. One of the replies there says thinkpads have the same problem, and I have an IBM power adapter, so that could be the culprit. II

My toughbook does this as well. One of the replies there says thinkpads have the same problem, and I have an IBM power adapter, so that could be the culprit. IIRC it's high frequency, so probably created by the supply itself rather than a floating neutral in the building wiring.