Belt squeaking?

[LSNE]

I was wondering if anyone can help me with what is possibly a squeaky belt.

When driving along everything is fine. As soon as I take my foot off the accelerator, it sounds like a belt is squeaking. When I put my foot down it goes.
It also disappears if I press the clutch in.

I've had the water pump changed along with all the belts last year.
2017 T6 150.

 

[Grim Reaper]

When they changed the auxilliary belt (the ribbed belt that runs the alternator and air conditioning compressor) did they change the tensioner as well?
This is the usual cause of chirping on idle.
Latest version of the tensioner is part number 04L145299 M