Help With Screeching Noise - '05 Daytona 650

I'm hoping the following link to the video works -
https://photos.app.goo.gl/13GpjuqFEwoTFiEYA

So this noise presented itself a while back but much quieter and only after being ridden, not at start up. The bike has been sat for some time, and for the last 4 weeks (as I've been away) has been with my local mechanic to have other issues rectified and has been serviced and MOT'd. He has no idea what it could be and has advised me that it could take some time, be relatively costly and financially un-economical to figure out the source.

My first thought was the fan as it sounds like something spinning, but as you can tell from the video it's coming from the rear of the engine and gets quieter as I move my phone towards the front.

Any ideas?
I'm not much clued up on bikes so any help would be appreciated, thanks.

 

Does anyone have any clue?

I've been told it could be caused by the cam chain tensioner being too tight, but the noise wasn't present when I first purchased the bike, and I haven't nor has anyone else tightened the CCT while it's been in my possession.

Searched the web high and low but cannot find anyone with a similar issue!

 

Did you manage to remedy it?

 

https://photos.app.goo.gl/8NWP7QY5miVxHDak6

Video of the noise whilst riding. Seems to match the rev's when I'm off the throttle and dissappears when I'm on it.

 

I've had a look on a few part sites and it would seem the sprag clutch for my bike is no longer available. I've come across this post https://www.triumphrat.net/threads/02-07-s3-daytona-sprint-sprag-clutch-replacement-option.746130/ where they've recommended using an OEM Hayabusa sprag clutch assembly and swapping it over into the old Triumph housing will work. I'm not mechanically sound enough to do this myself. Any idea how much time it might take a reputable mechanic to get the job done?