Could be one of 4 things:
1. (least likely I think) The motor needs a drop of oil to lubricate it.
2. The belt is riding up and rubbing against the belt guides. Sometimes this just occurs on start up and slow down or when the top plate gets a little wobbled or knocked. As long as it doesn't continue throughout play it should be OK. You can check this by turning the outer platter upside down, balance the mat and a record on top of this and start the table. You should see the belt rise up and touch the belt guide initially and then slip down again. You could adjust the suspension on the top plate so it doesn't do this.
3. As above but continues through play rather than just temporary on start and stop this is because the top plate suspension is a little misaligned. You can check for this by doing the upside platter trick again or by putting a record on with the platter right way up (but no sound) and just very gently put a slight pressure on the corners of the top chassis. If the grinding noise gets louder or stops when you press on a particular corner it suggests you need to fiddle with the suspension a little more.
4. The top plate is riding too low and catching the top of the motor pulley. You can check for this by doing the upside down platter trick again. If it is then adjust the suspension again.
That's the best I can do I'm afraid.