While I would not want to stifle any well intentioned creative thinking about possible indicators like acoustic signatures, they perhaps border on being a red herring here.
The elephant in the room is the AS332L2 was certified with magnets to catch epicyclic debris such as spalling, and the Maintenance Manual (see AAIB report App C2) called for them to be examined after any chip indication on the epicyclic mag plug. The mag plug and the magnets were specifically fitted to give early warning of failures.
According to AAIB they were never examined on G-REDL, on 25 March 2009 or subsequently.