Help. Humming sound on cold start
 

When starting my 1999 Boxster during cold start (when the car has been sitting for 2 or more hours) there is a humming sound located behind passenger side on right. This humming is loud as if a small motor or pump is running, then shuts off after around 1 min.

Does anyone know what this may be?

Has it always done this? It sounds like the electrically-operated secondary air pump, which normally runs for a specified time following a cold start.

Maybe it has but I,ve only noticed it in the last week and a half. Could be the pump is just getting louder because of age. Is there a secondary air pump in that location.