I found out: The removable filters on the front side that can be pushed in are a bit loose and produce the noise. Fixed it with some tape and now it's gone.
Unfortunately, we currently have no information about when the item will be available again and when we will be able to deliver it. The supplier himself is still waiting for a subsequent delivery from the manufacturer and for this reason he had to set the delivery date to unknown. As soon as something changes in this regard, we will post it here. Sorry.