The grim discovery in an apartment block in the eastern city of Strasbourg was made at the weekend after the neighbour called police about the radio which had been at full blast, day and night, for two weeks, reported regional newspaper Dernières Nouvelles d'Alsace (DNA).
When police entered the apartment, they found it filled with pigeons which had got in through a window. They believe one of them had switched on the radio by sitting on it.
The man's death had not been alerted to any authorities as his rent had continued to be paid by direct debit from his bank account.
Police said the death of the man, who was born in 1931 and died aged 78, appeared to be from natural causes.
"People don't really known each other much," said one resident, reported the DNA newspaper.
The man had lived in the apartment block since 1976, but those living there change frequently. "You only really know your immediate neighbours," said another resident.
The head of the company that runs the building told the newspaper that he was shocked by the death.
"I've seen cases of people being found one or two months after their death, but never three years," said Bernard Matter.
"We will look at other things we can do such as an alert when there is a very low consumption of water."