Here are 2 links to get you started. I think the 2nd link's idea is better if you have access to the joist and sub-floor from below. It enables you to 'pull' the floor down. Just be sure you don't create another problem in doing that!!!
You can try wood shims between the sub-floor and the joist, perhaps some caulk can be forced between the 2 pieces.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!
© 2016 Renovate Your World LLC