Not just a wall socket
We changed house at the end of 2017. The previous owner of our new house was - apparently - an engineer, and had a (very) high opinion of himself. He even bragged about having done all the electrical stuff himself. When we moved in, we discovered a few quirks that we had not noticed before, notably a slight buzzing sound in the ceiling of the entry hallway. We didn't make much of it at the time, it wasn't loud enough to be annoying.
Some time in 2018 we discovered that the electric heaters in one room were defective and needed replacing (not something you notice when visiting in the summer). So we shopped around for replacements and found something to our liking, which we got installed in August. When the installer came, he asked to look at the electrical cabinet. I showed it to him and, on opening the panel, I saw his expression change. He looked at me and said "Sir, I cannot install your heaters with this mess. I'm going to have to redo your panel entirely."
He explained carefully and clearly why the existing installation was not only defective and obsolete, but could very well be source of serious trouble (ie a fire). I obviously gave him permission to do everything he thought necessary.
Once everything was cleared up and the heaters installed, the buzzing had disappeared in the entry hallway.
It would appear that the previous owner was less competent than he thought.