Rattling noise at approx 20 mph while accelerating on 2010 Toyota Highlander

Hi, all…seeking input! 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, thank goodness.) 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). If I can catch the sound with my phone I’ll attach a sound file in a bit…in the meantime, thoughts? Thanks so much!

Could also be something loose inside a muffler.

One thing to try is to remove everything from the trunk, including the spare tire and associated hardware. Make it clean as a whistle back there. See if the rattle goes away.