URO aluminum water pump pulley, $20. It's worth it.
Check your tensioners and idler while you're at it.

IMO, the cooling system on an M54 needs attention/maintenance in this order:
- expansion tank, replace it with Genuine BMW or Behr; use an E30 rad cap
- water pump, several brands, none of them have plastic impellers anymore.
- thermostat, Wahler is the OEM
- belts/hoses; Contitech or Rein; brass screw for upper hose bleeding
- tensioners/idler; INA
- rad, if more than about 150k miles; Behr or Nissens

Most guys address all of these in one shot, except maybe the radiator. If you are replacing the expansion tank, you will also want to order a trans thermostat; they rarely come out in one piece. Sometimes they don't want to come out at all, lol.

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12 E70 3.5i xDrive
03 E46 330i ZHP
01 E53 3.0i
98 E36 323is
12 Audi A4 Quattro
79 Triumph Spitfire
73 MGB