I have read that bearing noise sounds more like someone hitting the inside of the block with a hammer.

This sounds more like a tick-tick tick like nailing.

How long do you figure that can go on before failure? (I ask b/c it's been like that for >3 years and >50k)

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT