Could You Add 3/4" Plywood to Fix Floor Rattling

That might work, and here is another thought: Many hardware stores carry tapered wooden shims about 9" long, 1-1/2" wide, 1/16" thick at one end and 1/8" thick at the other. They can be tapped into gaps, with or without glue, to keep floors, walls, or stair steps from rattling.

No, I'm too busy at the moment, can't you do it yourself?

But seriously, your suggested fix will reduce the flexing in a joist and if that's what you mean by 'rattling' it will do the job, it may be quicker, simpler and cheaper simply to bolt a similar joist alongside the first one. But if you have a ready supply of 3/4" ply then go for it.
Solving problems is easy the hard part is identifying the actual problem.
If it really is just rattling, then screwing the offending floor boards down tight to the joist will do prob the job, a nice quick fix may be achieved with the dreaded squirty expanding foam if you can find exactly where the rattle is emanating from, this is handy if it's at the end of a floor board where it may overhang unsupported, the foam can key it to the wall/other boards/skirting/your hands/the cat etc.
Del

Yes you can. This is commonly called 'joist sistering'. Usually done with dimension lumber (which will work better), but as mentioned if you have a bunch of plywood, it will probably help more than it will hurt.

Jack up the sagging joists before adding the sisters. A very slight over-camber may be in order... but not too much.

Thanks you all it help me to go a head with it. Thanks again.

Generally, the reason why your floor squeaks is the across-the-grain shrinkage of the timber floorboards which allows gaps to occur between your boards and the cross timbers.

the best thing to do is take up the floorboards and replace them, but I am guessing that you have access to the underside and hence your solution.

I would therefore agree with the idea of gluing in thin slivers of hardwood. Make sure it is seasoned hardwood so that it is strong enough to take the load.