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.
As desperate as this sounds, in comedy, 'likes' and 'views,' 'friends,' and 'retweets' really help. If I've ever helped you, please help me by friending, liking, viewing, and retweeting. It makes others think I have a following...it'll take you a minute or less. If you could, I'd really appreciate it!
A recent set--NSFW audio: https://youtu.be/_dcxcZfD6dg
Mangofy your E46!