Strange noise when in 3rd gear??

Can someone please help??
My 1973 vw bay type 4 engine has started to make a noise when in 3rd gear..
It sounds like it's coming from the rear right hand side and it sounds like a whooshing / rubbing noise??
It only happens when in 3rd gear and accelerating??? Every other gear is fine.
I've not used it much over the winter and everything was fine before putting her away?
Every other gear is perfect it's just when I accelerate in 3rd gear??

Any ideas?

Thanks

 

Sounds like your gearbox is on its way out? Check the oil level,you need a 17mm allen key.