What you need for seeing Mains noise is an oscilloscope. There you spot any dirt in the mains visually.
Alternatively, you can try a mains filter device. Then you hear right away if it is a mains pollution from outside.
Those come in various builds depending on the Power you need. For testing just lend a small one…
It is just a relatively simple circuitry, mainly of coils and capacitors.
If it is not coming through mains it could be radiated from nasty machinery or transformers, in the vicinity.
Then that would be a real problem… Not likely, though.
Servus, Big