I'd say it's a bearing going out on an idler pulley, or a water pump. Pull the belt(s) off and give each accessory a spin to see if you can locate the problem child.

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2001 X5 Sport 3.0/5-speed
1998 318ti/5-speed
1988 735i/5-speed
1984 528e/5-speed (soon to be M20B25-powered 525i!)