if the dealer tells u this is normal - they are lying - cause in my neighborhood there are a gzillion pacificas, three on my block alone, and i drove a few and they dont make the sound, it sounds like the electric vacuum pump for the power breaks which is in that area, that goes off when the vehicle speed sensor value goes to 0, for many years now, the Vehicle Speed Sensor, "VSS" signal (created by a hall-effect transducer on the transmission) gets fed around to all the computers including the speedometer, and in this case i think it tells the pump when to stop,
the weird noises the car makes when auto-following the car head in start stop traffic with adaptive cruise - those are normal, and using a different breaking circuit, and the transmission is doing rapid down-shifts so lots of noise making mechanical things are happening behind the scenes
by the way : there is a dangerous bug in adaptive cruise - if ur tracking a vehicle in front and u get cut off the computer picks that up perfectly OK, and if the vehicle right in front leaves ur lane it will track the next one perfectly, but only as long as its moving, if its stopped, it will plow right into it!!! even though if u were tracking and it stopped u would stop also , a really dumb bug!!!!