You probably lost the vital washer, that goes between the freehub and the wheel hub and is easy to overlook and drop on the floor. The freehub sits on a plain machined boss on the hub and relies on a consistent film of light oil to prevent it from wearing the soft alloy boss. The boss has grooves machined on it, which are supposed to encourage the oil to move around. There's a soft rubber seal that keeps the oil inside. There is only one bearing, a small one on the outboard end of the freehub but the inboard end is effectively floating on the oil film. It's a simple, effective design but relies on that oil to keep it working sweetly and that means regular stripping, cleaning and re-oiling with something like 2 in 1 oil. I tried car engine oil thinking it would do a better job and it certainly took up the slight play in the freehub on the boss but it was too viscous and the freehub dragged when freewheeling.
If you neglect the maintenance the oil becomes black with metal wear and the freehub body begins to scuff and wear the boss and eventually when the gap becomes big enough the freehub begins to oscillate causing a howling noise. Some blame this on the plastic washer set in the rear face of the freehub, on which the oil seal bears but I believe simple cyclical oscillation of the freehub on the boss is the reason.
@Yellow Saddle is a proper engineer and will be able to explain this oscillation better than I can.
The good news is that Mavic freehubs are easy to dismantle and if cared for will last as long as the braking surface. As the wheel ages, the alloy spokes will oxidise and the rim will wear and one day you'll realise it's time for a new wheel. Is this designed-in obsolescence? With a traditional wheel you can carry on rebuilding with new rims for as long as you want and with cartridge bearings you can continue to use the hub for years as well but a Mavic rear hub only has a finite life, as far as I can see.
