Hi Jacqueline, one of the many things I work with are new home warrenty issues. In this case I deal with squeaky floors on a weekly basis, and it can be a real pain trying to isolate the exact cause of each noise. The most common cause of the squeaks is simply not enough screws in a given area or the person that did the original screwing of the sub-floor missed some spots (easy to do). Also one thing I've found is that some local builders only use 1-1/4" screws which only leaves 3/4" worth of screw to catch the joist below (not much room for error). In doing the corrections on this type of situation, I use 2-1/2" screws so I have a better connection to the joists and I angle the screws so if I'm a little off I still can make contact with the joist.
Now, you mentioned that the squeak is originating between the joists. Sometimes, getting the adjoining joist screwed correctly will fix this, sometimes not. One possibility is that the builder had to butt two pieces of plywood together instead of them being tounge in groove, this would result in the two pieces rubbing against each other making noises (this would also explain bouncing -if the bounce is between the joists-).
Now for some more likely scenarios. Could the sounds possibly be traced to duct work in the basement? Could be a water or gas pipe secured too tightly to the joists. Finally, one that always drives me nuts.... check the steel I-beam for nails that are hooked around it. These (nails) if loosened a little will quite often get rid of those squeaks that the builder can't seem to find.
Hope this helps, and drop me a line if you need further assistance.