Pulled up some tile in the entryway, as we’re putting LVP throughout. I’ve got a squeak in one area of the subfloor (3/4” thick) I can’t get rid of.

Tried backing out and driving in existing screws, drove in additional screws next to them, still have a squeak. I can walk along the floor above the joists to either side of the squeaking one, and no squeak.

I have access from below, and the subfloor is also glued to the joists. Should I check for a gap between the joist and flooring? I did see a product that screws into the bottom of the flooring, then has a threaded rod that attaches to a hanger that hooks under the bottom of the joist, and you tighten up a nut on the rod to draw the floor down to the joist. Anyone try that?

If the subfloor is indeed glued down, what's squeaking? Pull the screws out and see if that changes. Can you put a chisel or prybar between subfloor and joist? If so, glue isn't gluing. Try to get some pl400 between.

Go ham w/ screws or, which I'd prefer, nail gun w/ 2-1/2" #8 ring Shank, about every 4". If it squeaks after that it's where the joist meets beam or wall.

Eta: if there's power, plumbing, etc running through those joists, make sure their anchors aren't squeaky either.

You could sister a joist running parallel with the existing tji/floor joist. Glued and screwed of course.

If it has wooden bridging (x bracing) between the joists, make sure the two x braces aren't touching each other in the middle of the x.  They can rub together like cricket legs.