Engine service now, loss of boost

Hi 

I recently purchased a 2015 fiesta zetec s black edition. Very happy with the car, good service history and receipt of wetbelt so all good. Shortly after purchase however, I noticed a sort of spinning sound while coasting (foot off the accelerator, clutch up) which disappears when I accelerate. Not noticeable with windows up, but It was annoying with windows down and I planned on taking it to my local garage to get checked when I got payed. However, driving home from work last week (shortly before payday!) I was accelerating after exiting a roundabout, in third gear, I felt the boost kick in and shortly after I had a loss of power and the ‘engine service now’ warning appeared on the dash. When I switch the engine off and back on again, the warning disappears and my power (boost) returns.

I took the car to my brothers who has an OBD scanner (snap on I believe) that he borrowed from his work (a Renault dealership) and there were 3 fault codes. Unfortunately, I didn’t make a note of the exact codes. I will update this post tonight with the codes as I am taking it back to his tonight to scan it again as the issue has happened again. 1 code was referencing the knock sensor. Another was referencing a turbo boost. 
 

I was just wondering, as we are able to access the knock sensor quite easily and the part is quite cheap, would it be worth changing the knock sensor to see if this fixes the issue? Also, side note, when locating the knock sensor, we noticed the car doesn’t have an under tray, which leaves the whole engine bay from underneath very exposed….does anyone know if this car has an under tray from factory? 
 

Thanks for any help