the e90 has 30k miles on it and is an auto.

There is a faint noise on idle (disappears when driving) that can only be heard if you turn the radio off and listen for it,

It's a little shudder / rattle that sounds like it's coming from the upper left hand side of the engine bay. It's also not constant there's a 1-2 second gap between each sound.

Revs do not change and idle perfectly, car drives perfectly also.

Is there some kind of pump in that area that could be getting noisy at that mileage? Or is it more likely that it's just something loose hitting off something? Engine mounts also spring to Mind but on 30k they couldn't be gone?

No warning lights, service due light is on since I bought it (5 days ago) and it's been booked in for Saturday week since the day I bought it.

Anyone ever had this before?