My 2014 Toyota Sienna XLE AWD makes a droning/vibrating sound at low rpm. 
My guesses are that maybe my license plate was not that tightly screwed in, or the exhaust pipe was vibrating against something, or even the exhaust heat shields were vibrating. I checked and tighten the plate, checked all the exhaust heat shields, and checked all the exhaust brackets. Everything seems solid but droning remains. 
I discovered a TON of forums with toyota and lexus owners complaining about the exact same thing but could not identify a clear solve. Some replaced the exhaust manifold, some tinkered with the CAT converter spring nuts, and etc etc etc. 

any thoughts or suggestions? 

Thank you

Marcel