I also have a slight whining noise in my engine bay which my mech thinks is the smog pump. The noise doesnt occur all the time though. I have changed all belts and still same noise sometimes. Noise goes away at acceleration or high speed.

__________________
2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6