BUZZING NOISE

[pumpaction]

as i have finally managed to clear enough space for another car in my garage i have finally started to put the s3
away at night and have noticed a low volume buzz coming from the bulkhead area after i switch off,continues for a couple of minutes then dissapears.its definitely electronic,a bit like the hum from some flouro lights.
can anyone tell me what this is.
thks in advance

 

[ahtcap]

Not sure if the s3 has 1,but i had a buzzing noise from the bulkhead,this was the culprit,6Q0906625.
2005 bkd tdi

 

[pumpaction]

not sure what this part is for,have looked it up but cant find a description of its function...thanks anyway..
anyone else got an idea.

 

[Bloxham]

Coolant run-on pump ? (got one on my old TT, not sure whether your car has one though)