What are the symptoms of IMS failure?

[RS964]

Hi All. I posted in the 996 forum but I've always had better luck here so here's my issue:

Let's say that the timing is still intact and the car still idles very well and has no apparent noises coming from the engine. My father's 996 with 50k miles has a noticeable vibration when revving the engine. Upon visual inspection of the drivetrain, we noticed that the tranny bellhousing has a crack all the way around. There is a very noticeable vibration when the engine is revved past 3k rpm. Im leaning towards an IMS failure but the bellhousing has me stumped. So either the motor or the tranny is done. I suppose one way to tell is to remove the tranny and run the engine to see if the vibration is still there. Any input? Thanks.