Thank you for your replies. I will loom into your suggestions. I really hope that it's the fan shroud. I will put some light onto that hopefully tonight. Unfortunately the sound occurs only when the car is driven so it is difficult to pinpoint the source it it's interference due to engine mount deflection under load.

I'll check the engine/tranny bolts. Strange if they decide to vibrate loose after 250k mi.

I had checked the air filter before; I'm confident it's tight. Where are the flex-disks and is there a test fo runout etc?

I have plans to replace the exhaust with a TimeValve S.S. system very soon. That should eliminate the exhaust as a suspect.

ConnClark, was the crank failure noise intermittent for 5+k mi with increasing frequency; or was is a click than break deal? Can you describe your symptom in more detail?

I'll listen to my IC more carefully, but am pretty sure the noise is comming from the engine compartment. It's loud enough to be a serious interference slap rather than a gear slip.

I'll inspect the fan shroud tonight for evidence of fan contact.

Thanks everyone and hopefully I'll find a cause before something more serious occurs.