Sienna motor stutters

My 2000 Toyota Sienna has 175,000 miles and for the past year has had an annoying problem. When you are on the highway or freeway, about every 1.5 minutes you hear a low chirp in the back and immediately after the motor loses power (stutters) for about 0.5 seconds. It’s a slight stutter and the car doesn’t lose speed. At night the headlights dim slightly when the car stutters. This happens in cruise control or not. When I took it to my mechanic the car didn’t stutter when they test drove it , so they couldn’t solve the problem. Any ideas? Is this something I should be worrying about? Thanks.

Are you sure the sound comes from the rear? Do you have the heat/AC on while this is happening? It sounds like an AC compressor going bad, to me, and when it cycles on, it throws a lot of drag on the motor (and makes the serpentine belt chirp).

I think you are right. I’ve determined that the chirp and stutter only happens when the fan is on, regardless of the heat setting. I could just not use the fan, but that would leave me without the defroster in the winter and air conditioner in the summer. Now I have some better information to give my mechanic.