grinding under acceleration

Hi all,
Well it finally started to drive properly, a bit more power, i can hear the turbo breathe and it generally feels good. UNTIL......
Yesterday, I thought I heard something..today I can.
When accelerating I can hear a grinding noise coming from underneath. It doesn't do it out of gear, revving the accelerator, only when I press the accelerator. Its a 94 300tdi manual and still under warranty.
Any help please.

take it back to the garage you got it from an get them to fix it if its still under warrenty.