I just put a motor in a '97 Cav for my daughter to use that I got out of a junkyard. It has like 180,000 on it. It does the same thing. I figure it is just old with worn bearings with extra clearance and such. I don't give it a lot of concern. If I was worried about it a lot I would just hook up a mechanical gauge so I could see exactly what it was doing. As long as it's only at a slow idle I wouldn't worry. Back in the day it was a common thing with a lot of Fords that had a lot of miles. I guess the oil pressure units had a pretty high rating to turn them off. Chevys wouldn't turn on until the pressure was almost zero.