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There's always a little slag on the differential drain plugs.

You may need to get another person outside the vehicle to help you further localize the location of the sound.

Is the brake squeal during freewheeling or during brake application? The parking brake is like a mini drum brake, there could reasonably be wear material or dirt/sand in there.

 

Freewheeling at slow speed.

I'll drive it around town this week. Maybe pull the wheel off next weekend. I am fimiliar with the GM parking brake. The toyota is probably similar. I would have never thought it could grab that hard (if that is what was happening).

 

Probably the best way to figure it out.

 

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