Spit boost hoses can cause this sort of sound, if the split is just the right sort , it wont suck air so the engine wont throw any emission codes. I would think you might notice the car a little less powerful. You could do with a Bentley dealer or Indi doing a boost leak test on the system.

As to the ECU it runs adaptive values, it will pull timing very quickly if it detects detonation, but takes many engine cycles to re introduce timing back.
So for eg, if your car has always ran on UK 98 ron fuel then you decide to run 93 Ron there is a distinct possibility that the ECU will pull timing to a safe level and you can run 93 for there after.
You will have less power on WOT and the car will be less responsive and use more fuel. However if you then decide to fill the tank with 98 again, the engine will remain running with the 93 octane adaptive values for quite a long time. It re introduces timing and boost very slowly and can actually take quite a few tank fills of Gas to get back to where you were before you ran the 93 octane.
So its a waist of time just running high ( Race) octane fuels without a tune to go with it as even adaptive values have a high set point at which the standard tune will not exceed.

 

Thanks for your explanations, will check on the boost hoses..

As for the RON usage, i think you are right. Coz my previous owner might be running on lower ron. When i first got the car, it gives very little distance even i use higher ron, then after few tanks i started to get more miles from it. Now after 7-8 tanks of higher ron, i get around 15% extra miles.I think the ecu has adapted to it like what you have mentioned.

On full tank, i'm able to run 280 miles, is this FC normal?

 

That's what I always get. Would love to see 300 plus.