Problem Solved! The sound was coming from the area in the trunk, on the driver's side behind the fuse bod: some sort of hard drive or similar device. I pulled two fuses and left them out overnight, then put everything back, and no more problem plus the radio started working again.
I think Fuse 6 did it.

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe