When you do an engine (general visual) inspection, pay attention to the mounting flanges of the transfer gearbox. It happened more than once that one or more of these mounting lugs where cracked. Also the bolt(s) can be broken. Some evidence of broken lugs, and therefore a TGB that can move a little, can be seen by oil traces coming from the driveshaft-coupling between the TGB and the AGB. Also metal parts found in the TGB/AGB chip detector (the fwd one) shows that damage occured and further investigation is necessary. See the 5 pictures below for more information.