My van loses power and the Power reduced message appears on the dashboard.

When driving,I accelerated and an obvious hissing noise started under engine/transmission.no power loss,no warning lights,no shifting problems. I went under truck,but could not pinpoint the source of… 

Gearbox locked on 4th gear what could be the problem

Did it harm the gearbox? The car is still changing gears but on taking off it loses power slightly

Loses power while I am driving