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drove in parking lot, and stop at stop sign, the brake make some grinding noise when I tried to stop, the feeling is almost like there is no pad/shoe or some component is missing or loose, but it's not the loud/sharp metal/metal grinding.
1) it only happen when drive in parking lot (straight, 10-20 mph), and some times there is no such sound, everything looks normal
2) then drove on the road (40mph) or highway (70mph), the brak function perfectly. I can stop at light or slow down. But once I drove to next shop and do some shopping, then drive again, that grinding noise come back, then once on road/highway brake feel normal.

and the grining noise only happen when I brake and when drive on pakring lot (low speed).

BTW, my rear ball join need to be placed, it make some rumbling noise when road is tough, shop giave me estimation around $170, I might fix it this time.

Question: what could cause the grinding noise (at low speed)?

Thank you!

 
Rep Power: 244

You probably only hear it at low speeds but I would have the brakes inspected. Grinding noise that is only present while braking is usually brake related.

When were the brakes last done?

 
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it's done about 1 year ago, replace the caliper and pad on front right side.
one more thing, last Friday when I drive on parking lot, ready to stop at the lane and I feel I hit a bump, and I know there is no bump at that spot, I look around nothing suspect at that time. Not sure if this is related, I will bring to shop to take a look.