Thanks to all. I examined the foot, found no thread or lint (even with a magnifying glass), but could easily see the spring, the hinges and all the moving parts. I used a drop of oil on everything I could see moving when I manipulated it by hand, and no more squeaks! I let it set for about half an hour (don't know if that was necessary), then used it to sew for several minutes on several layers of scrap fabrics to remove any excess oil, then left the fabric in the machine with the walking foot down and left it overnight. This morning everything is great. No oil on the fabric and no more squeaks.
Judging from my experience, I would not hesitate to advise anyone to oil a squeaky foot with moving parts.
Thanks for all your opinions and advice. I really appreciate it.