My understanding, and I must add that I am no expert, is that some power supplies use the frequenzy of the mains as type of clock as a reference for stabilising the rotational speed of the motor so that it accurately runs at 33/45/78 rpm.
The problem here is that the mains can fluctuate depending on the load, ie how many factories and homes it has to supply power to. The greatest load is during the day and at meal time when everyone is cooking, watching TV, running the air conditioner and using their computers to browse Audiocircle.
Many people report that their music sounds so much better later at night. This can be put down to a lesser load on the power supply or that they had a really great dinner.

Bluesky