@cleeds : Here in Poland +/- 10% is allowed, so up to 253V is "OK". Technicians were called, checked the voltage and confirmed the problem is with excessive local production by PV installations. Power lines will need to be replaced which will not happen any time soon. In case of any damages excessive voltage can make, they pay for repair. I'm monitoring my installation with Supla system and Zamel monitors and all the anomalies are recorded.
@tonnesen My installation consists of two strings, 6 + 4 kWp, oriented east-west. Fronius inverter I have, stops working in case of 254V+ measured in any phase constantly for a few minutes, then checks back and if voltage dropped, starts again. The inverter stops working and not individual panels or strings of panels. Your installation must be different. Sure there will be more problems if more installations will be connected, here and where you live, if no new lines and perhaps auto transformers will be installed in the local network.
No hum at night when voltage drops to 230V or below.
@baylinor I have a written warranty from a local-private importer so it is not that I am not covered. Starting this discussion I just wanted to be better prepared with my claim. If I get any responses from the members (especially from EU) that the transport is DEAD quiet (in the casing) at any excessive voltages, it will be strong indication that my unit needs repair. Another possibility is that all units slightly hum but their users are not bothered... Funny is that the Berkeley Audio Design HDCD DAC of the importer hums like hell (by my standards at least), but he likes it so much so ignores the noise. "Unfortunately" at his place voltage is dead stable so my problem could not be replicated.
You said "no clue what EU power units may sound like" but I don't hear the power unit which is on the left side but something buzzing at the right side. Is there anything that could vibrate there resulting in 300Hz noise?