The intake manifold has specifically designed runners, ie a certain length. When designing the engine, they would have had to pick longer or shorter, and each has a different 'sweet spot' in terms of engine rpm that is best for power and torque. As with many other modern engines, the M54 engine has a dual intake tracts, of different lengths, and a valve that switches over the airflow from one to the other depending on engine speed. It can be called a resonance valve as it is using resonance to achieve the tuning benefits of the different length runners. Also called a DISA valve.

It is located under the plastic engine cover on top of the engine.

Number 7 in this parts diagram:

RealOEM.com * BMW E53 X5 3.0i Intake manifold system

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue