Hi, after changing air filter i also find small sand-dust in the air filter box. Before I thought it was because of dirty filter, but now it is clean! Is it normal? because my Audi's air filter box is clean, whether filter is clean or dirty

What side of the filter is this dirt? There shouldn't be anything on the engine side.

I get that in my car too and also the occasional flies. My friend at lexus say they get the old vacuum cleaner out and give the box a once over during servicing!

If the Engine side is right, then the dust appears on the left and center parts of box. I think

And Yes, vacuum cleaner helps, for some period

yes it is quite normal its just the dust/dirt that is in the air that has collected

I would flush it just in case.