Time for new brake hoses, they are cheap. Check the retration of the pads, you should see them visibly move back when the pedal is pushed down and released. Very small movement, but they should come away from the rotor.

If not, rebuild or swap, the seal is getting hard and no longer pulling the pads back. This will cause them to drag and generate the hum from differential loading on the rear wheels.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!