Check the secondary air injection pump. Mine howled for a few seconds at start up. Ended up going through three of them. Two low cost ones made noises too. Ended up with RockAuto's most expensive air pump to stop the growling. You can unplug it and put 12V on to test it.

__________________
'06 X5 3.0i - bought @143,123 miles (12/26/20)