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That's ABS.

I thought ABS was generally activated if braking really hard from high speed, but I hear the weird grinding somewhat consistently when lightly to moderately tapping the brake while going < 30mph.

I do know (or at least, I think I know) what ABS feels like when being applied and it doesn't feel like that. But my frame of reference is much older cars before the 2021 rav4 prime I have now, so maybe it's just different.

I don't know your car specifically. But newer ABS may pulsate at a higher frequency than older systems.

But with a PHEV, you might also be using regenerative braking more often and friction brakes less often? If so, the friction brakes might have more corrosion and give some odd sounds when they are finally put into use, as they scrape the surfaces clean again.

It's also activated when there is slip detected, such as when you go over a broken section of road while braking or even turning. My Chevy Suburban will engage the ABS even without me braking.

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