So.

Fixing one issue usually reveals another.

When driving at motorway speeds giving full throttle makes my supercharger whoops for 0.5 seconds when start pulling. Then it quiets down and it whines like a supercharger always does. It's the whining sound but only louder and only when it starts to pull.

Little or moderate throttle does not make the louder whooping sound.

Now. I know there is a supercharger belt can it be this slipping? Does the supercharger needs service? I think I read somewhere about tensioners?

 

The supercharger does, I believe, have its own small oil reservoir so checking this might be helpful. Also AFAIK, the drive belt is toothed so it can't slip, (but I may be wrong on this). The superchargers have bearings that if they get too worn out, render the compressor scrap, as the rotor scrapes the housing and wears it out, thus making it non-repairable.

My technical notes copy shows there are two jockey wheels on the drive belt, one of which will be the tensioner.

 

My immediate reaction was ... belt slippage ?