Question about Dyson Pure Cool Link

We have two Dyson Cool Pure Link. Both bought shortly after each other. After about half a year, they both start to make a strange noise while ventilating - as if a thin piece of plastic is hanging in there somewhere. Nothing to be found when opened. Since one was in the office away from any snippets, it can't really be a foreign object either. Has anyone else noticed this? I'm just surprised that both developed the same problem shortly after each other, despite being 50 km apart.