We found the source of the 48Hz bump.
It comes from a WE motor crate, the one that drives the MH.MA.1 and MH.MA.2 motors. We don't know why this new coupling mechanism has occured since Tuesday, but removing the mains cable from an unpowered crate was sufficient to eliminate the noise.

We leave the power cord reconnected but the front panel switch in the OFF position (it is equivalent).