Strange noises, rearend trouble?
 

I took my car to work last night ('66) so I could leave to pick up another coworker from the airport , and I noticed a strange noise for the first time.
While on one of the cloverleafs at TIA I could hear sort of a "whirring/gear" sound kinda like a mid/high pitched vrrrrrr sound when turning. It seemed to be coming from the rear end, and disappeared when the car was going straight. I noticed it again pulling a wide 180 turn at 7-11 on the way home. Its not real loud, if the radio worked I might not have noticed it. The car didnt seem to drive any differently.
Is this normal? could it be because the car has an open diff? (havent gotten to check yet, but I know how)
I'm still unfamiliar enough with the cars "personality" that every strange noise makes me nervous. I'm just worried that the rear ends about to lock up on me or something.

Any help appreciated,

J