Awful grinding noise in High gears.

 Many thanks for letting me join the forum. I have an issue with my 11 speed cyclocross cannondale CAADX 105 which is basically in higer gears ( small rear cog in case my terminology is off) i get an awful grinding sound.

It starts in gear 8 all the way to 11 , intensfying as I go. The same thing happens when in the big or small cog on front.  I have exhausted all info on the internet hence the need to ask some experts.

Things I have tried to date :-

1)B-Screw adjustment , Fine no rubbing on the rear derailleur 

2) H and L screw adjustment , perfectly fine.

3) Rear derailleur cable tension fine, gear shifting is fine with no hesitation. 

4) Front derailleur not rubbing, I am very used to this having 105 and the trim function which makes a distintctive sound. 

5) Took the cassette off , reseated it and ensured fully tightened.

I ride all year round and in winter I fit studded tyres as I once had a painful ice accident, which I don’t want to repeat. With these tyres I never go to the higher gears so I have no idea when this could have started. With the slicks back on I need these higher gears.

So as mentioned I’m now stumped and am out of ideas….

Could it be a worn chain or cassette.

I’m reluctant to visit my LBS as it’s not so local and they ripped me off the last time I used them. I ‘ve managed all my own repairs except for this issue