ABS accumulator clunk

[ASUMTNEER]

I'm searching for an answer to a clunk in the brake system of my aunts 2016 4runner. It feels, and sounds, like a "clunk" in the pedal similar to when the abs system kicks in. It happens on the initial press of the pedal and about every 2nd or 3rd time you tap the brakes during a drive or sitting still. I was looking under the hood and noticed that the accumulator is moving ( I assume by building pressure) and is knocking into something that I can't see to cause the noise. Has anyone come across this before? Is there a solution? That thing is in a very tight space, so I imagine it will be difficult to replace if need be. TIA