Originally posted by SniperX
I guess they are either Comp hubs ( steel axle and freehub body)
or Xlite ( aluminium axle and freehub body).
As the noise only happens when not pedaling and 'freewheeling' the problem appears to be in the freehub body ....which is the part of the hub the cassette slides onto.
The normal noise you get when free wheeling are the pawls (which engage when pedaling) these when freewheeling drop over the notches that the pawls push against ( like steps) some times if the wheels have been idle for a while they can grate especially if the bike has been left after riding in the rain or on wet roads this sets up some corrosion or dirt may have got past the seals, this sets up a grinding noise. The body bearings also may have got water in them and the result is a grinding noisy hub when freewheeling.
Bont hubs have cartridge and needle roller bearings in the rear hub body. One way to test if there is water penetration is to flush something like WD40 AEROSOL past the seals under pressure..... spin the wheel forward holding the cassette and listen for a change in the noise. The freewheel body normally has seals which you can lift up to allow the oil to get in. The excess oil if contaminated will leak out the gunk and rust.
Get the hub stripped and freewheel body replaced or serviced... some bike shops don't go as far as stripping the freehub body and replace the bearings and save you some money .... they would rather replace the body.