On a M42 engine, its between the upper and lower intake. When you remove the upper intake, you will see it cradled in the lower intake. Its easy to take it out and clean it.
On the M44 its a different design attached to the front of the upper intake.
I think I have seen cleaning DIY's for both types in the knowledge base.
Edit... Links added.
M42 ICV Cleaning
http://forums.bimmerforums.com/forum...d.php?t=454501 M44 ICV Cleaning
http://www.318ti.org/forum/showthrea...+control+valve If this does not fix your problem... look into the Crank Vent Valve