Eesti BMW Klubi

If you have a failing secondary air pump, you may hear a whistling/growling/screeching noise, or no noise at all, from the pump during the first couple of minutes after a cold start. The bearings can make a racket if water gets into them. An undamaged pump makes a gentle whirring noise, sort of like a muffled vacuum cleaner, under the hood.
The purpose of the pump is to inject extra air (specifically, oxygen) into the catalytic converters to get them up to their operating temperature faster. They have a very high operating temperature (think several hundred degrees Fahrenheit), and getting up to temperature faster lets them treat the engine exhaust gases faster.