Happy update time!
I have replaced the cam belt tensioner bearings (moving and static) and checked the belt tension (vertical belt was tighter than horizontal but not overly tight) and the problem seems to have gone away!
I'm well chuffed fair play! I really thought it was the clutch and after I'd stripped that down and the sound was still there, I had a horrible feeling it was the main output bearing or something more serious.
It wasn't without issue though, I spoke to my usual Ducati dealership and was told that Ducati no longer supply the bearings, they can't get them anymore apparently. I panicked for a bit then remembered I have a shop in my town called 'Bearing man' (I kid you not) I rang them expecting to be told they don't do automotive stuff and the guy was like "Yeah mate, we've got over 100 in stock" he was really knowledgeable on the phone and when I read out the code on the bearing's I'd removed, he told me what the different letters meant etc, confirmed that they were high speed/temp bearings and as a further bonus they were only £8 each, bloody bargain!
The Horizontal bearings were easy to fit but the vertical cylinder was a right pain to change, in the end I had to find a short 19mm socket, grind two flats on either side so a 22mm spanner would go on it to remove the tensioner stud and backing plate off the block just to enable me to get the old one out and the new one in. Right pain in the backside. I'll update the post later to show the bearing numbers etc in case someone else with an S4 needs bearings and they don't want to spend £40 a pair on ebay prices. I'm a bit disappointed with Ducati, I know its not a new bike but they could of been a bit more helpful.
Thanks to everyone that made suggestions, I now have a bike that sounds fantastic again instead of a police siren on helium everytime I hit 3500 rpm.
Cheers guys
