Follow the hose from sap out...it goes to the valve...silver thing on exhaust side of engine, with vac hose leaving out the back...assuming, in the UK, you have this pump. The clue, so you know, is that you said the sound disappeared after two minutes...that's how long the SAP runs on cold start, plus or minus a bit.
Upcoming NYC dates: Jan 8 and 27. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!