My best guess is that the glue in my plywood subfloor is just getting old and giving up it's grip. As the plies move underfoot it squeaks against the nails thru it.

My house was built in 1986. It is a ranch with full basement, no ceiling in the basement. All dry and no signs of anything bad. No changes to use or humidity. The floors are 2 x 10 joists with 1" plywood subfloor. For about 20 years no squeaks, but since then the squeaks have started everywhere, except where there is 3/4" hardwood. The hardwood runs across the joists so it carries the load and distributes it. When we replaced the vinyl in the kitchen last year we added fasteners in the squeaky spots and it was quiet for a while, but then other spots spoke up. Grrr.

The bathroom has now started and I am planning to replace the vinyl there. Not wanting to try to replace the 5' x 5' subfloor I'm thinking to add 1/2"
Baltic Birch, after adding many fasteners to the subfloor. Might this provide enough stiffness to quiet the problem?