1991 190E 2.6

Some pump/pulley began whirring/whining a few days ago. I've narrowed it down to the smog pump and/or smog pump pulley.

Questions:
1. What are these things made of? Gold? $1,000? Found a rebuilt pump with pulley for about $350.
2. Typically, what causes this problem? Is it the pump or is it the pulley?
2b. If pump, can't I just disconnect the electric to the pump and be done with it?
2c. If pulley, how difficult is it to install a correct sized belt that bypasses the pump considering moderate to pretty good technical skills?
3. Considering same skills, how difficult would it be for me to r/r said pump or pulley?
4. How do I confirm whether it is the pump or the pulley?

-- It seems to continue even after the engine is warm. No matter how long it has been running, if I raise the rpms it is evident.

Thanks mucho for the info.

Anyone have links to the rebuilds mentioned in other threads and would the quality be ok?

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