Faint whine!

  1. My model of MX-5 is: NC 3.5
  2. I’m based near: Moray
  3. I’m looking for technical help or recommendations on:
    2010 NC3.5 2ltr. In 4th gear only, around 3000 revs and 40mph a faint but annoying high pitched whine starts when engine is on load. Reduce the throttle pedal, goes away, increase pedal, comes back. It’s hard to say but I think the frequency of the whine increases with speed but can’t hear it above 3500 to 4000 revs for engine noise. No idea where it comes from and can only hear it with the prht and windows up otherwise too much wind noise.
    Im hoping it isnt a gearbox issue but could well be ?

Any ideas guys? Thanks as always.