Floating in the air ?

[Fabrice]

You could move them around with some noise forces or something, then use a Push In/Out operator to push particles back in if they start to move out. Or could you use another plugin like FumeFX or PhoenixFD to simulate fluid (smoke) within the volume, then advect particles with the resulting grid. If the source geometry for the fluid is air-tight, there shouldn't be any leaking and the particles will stay inside.

Thanks, I tried to add a Force operator using an animated Noise but it doesnt have the expected effect :
the particles just move into the air in straight line
instead of moving according to the noise.