The 300 TDi has a sprung belt tensioner so there is no adjustment that you can make, it does it automatic like.
It could be a glazed belt or dirt in the grooves of the pullys.
With the engine running spray a small amout of WD40 onto the top of the belt, just before the tensioner pully and see if the noise stops.
If it does then, providing you don't have air-con, look down the line of the belt (from the offside wing) as it passes through the tensioner and alternator pullys and make sure it's alined correctly.
Sometimes the the tensioner pully can be pulled out of line when releasing the tension on the belt and a small packer placed behind the pully body will bring it back in line again.
If this is okay then try a new belt and see if it cures it.
If the noise doesn't stop when you spray the WD40 on the belt then take a close look at the centre bearing on the belt tentioner, as has been said above.