If the sound is actually the water pump rather than the tensioner (hard to tell the difference) it would make sort of a buzzing sound.
After R&R belt, pump and tensioner on my '91 100Q 5cyl last December it took to making a whining noise at idle when cold but quit when warmed a bit.
The belt tension is checked like you say with the 90 degree twist but even the it is a mater of how hard it is to twist. Another way I test it to look for about 3/8 to 1/2 inch of flex in the belt between the water pump and the cam pulley.
The bearings in the tenwioner are sealed and do not need greasing.
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