Good day!
Can you hear it really well with the hood up, trans in park? Your tech should be able to isolate the source within a short time! I would suspect an intake leak, dirty air filter, or a small vac leak...

Good luck!

Cheers!

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2001 E53 X5 4.4i (03-07-2001 build date) White exterior / hellbeige interior (6MT swap vehicle - Manny)
2002 E53 X5 4.6is (12-20-2001 build date) Imola Red exterior / M-texture interior (Red)
2003 E53 X5 4.4i (06-26-2003 build date) Silver exterior / Black interior (Silvester II)
2008 Mini Cooper S ragtop (04-17-2008 build date) Dark Silver exterior / Gray interior ( Topless)
BMWCCA member (#4745)