Sienna2008 clicking noise entering freeway.

Your Stability Control is kicking in. As you accelerate on a ramp, the vehicle naturally drifts towards the outside of the turn. If the drift exceeds the expectations of the yaw and pitch sensors, then the control system begins to dip the throttle and selectively apply a single brake or two to help make the vehicle 'rotate' and correct it's track. At 215k miles you probably have a lot of worn and loose front end components that give more 'play' in these turns, increasing the actual rate of drift, or the perceived rate of drift as viewed by the computer. Begin with a Zero Point Calibration procedure to recenter the Stability system. It's possible that it's just out of center. If that doesn't do it, then begin looking for a steering rack that shifts on the firewall (bad bushings) or other front end problem.

We have a 08 limited sienna with 154k that does this. Looses power when entering on ramps and exiting the anti lock breaks engage. Last week going 80 on interstate it went off beeping and anti lock breaks engaged for a second. That was our final straw. We called Toyota proper, surprise, no help. So before all that many visits to dealerships and mechanics no help. The only thing told to us is alignment is off. We have taking it in twice in 3 months to be looked at. This has been going on for 3 years with no answer. Now there is a constant tick in the steering wheel that just developed.

The steering angle sensor needs to have a calibration done. If you handy you can do this yourself or pay a shop to do it

I had a similar issue and it was the rack and pinion. Cost $1400 to get replaced but problem.