Normal turbo sounds should be "ssshhhhhhhhhh" at whatever volume. Any "tinging" or other metalic sounds are definitly bad. My brother's T25 died from bad oil seals (imported that way) caused by very dirty oil. It had no shaft play, but it dropped enough oil to leak from the downpipe gasket. Turbos that are going bad at the bearings will spool noticably slower and not make peak boost as fast or at all. If the bearings are really going, you'll probably hear blades dragging on the housings.