There are a couple methods of detecting failing bearing assemblies that were developed as preventative maintenance practice for factory and shop machines. I am anxious to learn what epapp has to offer the automotive world so I can go buy that 997 I’ve been keeping an eye on!
http://www.mobiusinstitute.com/articles.aspx?id=2088
Using accelerometers or using a broadband pickup and heterodyning for our ears to hear are interesting thoughts but I think would be a challenge on internal combustible engines (with all the other noises and moving parts) but if one could be installed on the IMS bearing cover it might work… OK, I’m off to the lab to develop.