2011 Sienna XLE Brake Grinding

A rock or pebble between the oad and the rotor.

A great suggestion. However, the mechanic completely took the assembly apart and a rock would seem to have fallen out or been discovered, right? You think another one could get popped in again?

And I should clarify that I cannot discern if it is coming from the front or rear brakes. It seems like it is the whole system. :s

Is this the ABS system working? Some people mistake the ABS pulsing for grinding.

I don't believe so. It is a loud, groaning noise accompanied by the grinding sensation. My wife says the car doesn't slow as well when this happens. This is on dry pavement.

I would get a second opinion from another driver.

I've had it happen to me (had to work hard at it). It isn't like prior experiences with ABS I've had. The mechanic said the same. We're a little stumped. Car as in in the 60-70k miles range.

Have you recently had brake work done ? Sounds like a problem I had after doing a brake job mistakenly got some brake grease on rotor and with hard braking problem you are having. Removed rotors and sanded the rotors with power sander or you could just replace them or have them resurfaced.

This is most likely caused by an issue with the transaxle if you have an all wheel drive model