got a noise on mercedes ml430 coming from gearbox area?

I have a noise which sounds a bit like metal rubbing on a brake disc but have had them all checked. Sounds as though its coming from the middle of car, auto gearbox works as it should pulls well with smooth gear changes. The noise is there in drive, reverse and coasting in nuetral, no oil leaks, noise is driving me mad! Would appreciate any info on this please.