There is a vibration on 2015 Sienna when engine speed at around 1250 RPM. Some vehicles are very minor, and others could be very severe. The vibration on my 2015 Sienna sometimes was very severe and along with the lower frequency drone noise. This had made me feeling very headache and ears pain. Had brought it to the dealer several times for a inspection. Nothing found! Had test another brand new 2015 Sienna SE, did the same thing, just a little better than mine. The last time I went to the dealer, they did something on the CAT converter. Before, there were four lock washer on the 4 bolts (with spring on it) those connects the CAT converter to the front and rear manifold pipe. The service tech removed those lock washers and tightened the four bolts. Because they believed that the vibration was caused by the looseness of those joins. But after I picked up my Sienna , I found the vibration was worse than before. Then I was thinking if I made it a little looser, would it be a little better?? So what I did is put 2 flat washer and 1 lock washer on each bolt. After I tightened those bolts, the spring on it will only be compressed 1 to 2 turns. I test drove my Sienna after doing this. The vibration almost went away. There is only minor vibration when cold start the car. Those drone noise also is not noticeable. If you are experiencing the vibration on your Sienna. You may try this on your car(you can put more or less washers on your bolts, just remember do not make the spring too tight, so the washer amount may vary on different vehicle). Please see the following pic for the washer size and position of CAT converter.