My uncle?s 2004 R5 Volkswagen Touareg makes a very strange grinding noise when he accelerates. It's sounds like a flat tyre, but it?s not. Some internet forums suggest it's the ABS re-calibrating, but he believes it has something to do with the differential. This mainly happens when he accelerates quickly.
It doesn’t sound like the ABS, it does sound more like the diff or something else in the driveline. I would get it checked by a VW specialist.