1997 Ford F-150 oil indicator

Here's what happened yesterday. The oil level was full. There was some white smoke at engine start up but not blue or dark smoke. No rattle as we drove to Sam's. Rode fine. Then on the way home, the rattle started and the oil indicator needle started quivering up and down and continued to do so while driving. The oil light came on at the stop lights and would go out once we accelerated again. The rattle sounded rhythmic like a train. We checked the oil level when the truck was cold again and the oil is still full. Does it sound like the oil is not getting to where it needs to go?