My '99 STS has 104K miles. I get an oscillation in the steering wheel when I make slow, sharp turns into a driveway. The fluid level is fine, I even changed it a few years back. I thought maybe the serpentine belt might be slipping. It's original so it was due anyway. It still oscillates. I did make note while I was changing the belt that I could tug on it and the tensioner moves easily. Is this normal? Could my tensioner be causing the problem? There's no squealing, just a subtle vibration in the wheel. Any ideas?