I think I have this same problem. I have changed the rear discs and pads as they were shot but I'm still getting it. Although the squeak is slightly different. Any ideas, maybe I should renew the shims as they were a little rusty and warped on the piston side?
Do prop shafts squeak?
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I had squeeking on the front of mine, turned out the calipers were seized.
On every rotation of the wheel it would squeak.
Jacked the car up and could not turn the wheel !
Got the garage to replace one side, then a few weeks later the other side need new calipers
In future I will try to drive round the Floods and not thru them lol !