*** My best guess is PCV ***
However, no substitute for good diagnostics. Is your CEL operating properly? If so, these are the 2 must-do diagnostic steps for your situation, IMHO:
1) Check for trouble codes. Borrow or buy a Peake reader, or count flash codes (one explanation of the process is here:
BMW Diagnostic Flash Codes: Bimmerzone.com)
2) Check vacuum readings. Use a gauge, don't just visually check for apparent leaks. Diagnosing via vacuum gauge is a wholly underappreciated skill. Shame, because it's simple, cheap, and very informative.
Good luck.
P.S. Quick question -- does this happen w/engine warm, cold, or both?