Symptom was a rumble in the engine at idle, turned out the engine fan coupling had developed a small play.
Bought part no 17417789256 from BMW, £210 with VAT!! ^$"%$^"£$%$, comes less than half price from ebay.. but naah, have to do it the expensive way!!
Tools needed: 2x32mm spanners!
Here's how to replace:
1. Remove the air intake and engine covers
2. Loosen the radiator cowling
Radiator cowling is held in place by 3 plastic clips, remove them and it can be lifted up. remove the box/sensor for the secondary radiator fan shown first. there's another relay for the fan clipped further left. TIS says to unclip it, i didn't as it seemed like i might break the clip. you don't need to remove it.
3. Remove the fan from the engine
1. hold the engine-end with a 32mm spanner
2. turn the fan-end with another 32mm spanner, turning CLOCKWISE. this will be tight. easiest is to give a nice tap onto the end of the spanner 2 with a rubber hammer, whilst holding spanner 1 against turning.
3. once removed the fan will fall into the cowling. it can't be taken out just yet because there's not enough space.
4. Take the fan out
1. Move the cowling towards the engine (cannot be fully lifted out as there's not enough space plus there's an oil horse clipped to it at the bottom).
2. pull the fan out between the cowling and the radiator
5. Seperate the fan coupling to replace
held by 3 hex screws.
Job done, reverse the whole procedure to get everything back in!
Never anthropomorphise computers. They hate that.
)