Toyota - Highlander :: 2010 - Rattling Noise At Approx 20 Mph While Accelerating

I have about 72k miles on my Highlander, which we purchased certified through a dealer in May. At the time of purchase, we noted an intermittent rattling noise, but didn't think much of it. It has increased in frequency and in volume since that time. I have had it to the dealership four times now to have it fixed. They have replaced the heat shield on the exhaust, tightened my license plates and today replaced my catalytic converter (at no cost to me) Imagine my dismay upon driving home and again hearing the rattle. When they decided the rattle was related to the catalytic converter, they were essentially banging around my car with the handle of a hammer and trying to replicate the rattle...needless to say, I am a little concerned that the techs are unable to determine the root of the problem, much less fix it. It seems to be correlated to some degree with a certain RPM. It sounds as if something is loose somewhere, but that place is a mystery to at least four different techs (all of whom I have driven around so that they could hear the sound and who still cannot identify it).