High revs while reversing

[Carrots]

2008 a3 Quattro s line
I am already aware that the inlet manifold needs a change and so I am pretty sure this is the cause of my issue - however just to be sure I thought I’d ask

When reversing after a long drive, I will stop the car infront of my drive and put it into reverse. I then notice that my revs are super high before the car starts to move backwards and it jitters - around 4.5rpm approx.

Could this be the manifold? (Getting changed sometime next week) or some other issue?

Thanks