Was told some time ago that pilots would not understand the hums, even if it was in a simple form, I would have thought there was a way to show discrepancy's on a display against the average of a previous period.
There is a way, but when flying over hostile surface such as N Sea, the consequences of a false warning creating a ditching scenario resulting in loss of life etc, is too great. The sensors for HUMS live in a very nasty environment with vibration, heat and dirt, and false warnings remain a problem.