I have the same problem on my '01 3.0. There is a post somewhere on here that attributes it to a crack in the assembly between the airbox and throttle body. I've been meaning to take care of mine for the last 12 months and haven't, no issues that I can tell except the sound.

Run a search and you should find a solution.

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2005 ///M3 Competition / Interlagos Blue-Black /6spd/SS Race Exhaust/Angel Eyes/Passport HW/Mich. Super Sports Soles (love em)
2008 ///M3 Melb Red w/ Beige Novillo, Tech, Prem, & 19" (Retired)
2005 X5 4.4i Estoril/Grey Sport, Nav, DSP, Prem, Pano, etc. (Wifey's)
2007 335i Silver/Black SP,PP,CWP, Comfort access, & Sirius (Retired)
2001 X5 3.0i (Retired) Silver/Gray Sport w/19"/Prem/DSP/CWP
2002 YZF600R (Retired)