If it's related to engine speed but not wheel speed, it's ahead of the transmission. I'd be looking closely at accessory drive bits or accessories. PS pump, alternator, water pump, AC pump...

Does the sound change with AC on or off?

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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!)