Okay, I thought about that again. And this is my conclusion: It is not Avast. When you click on the Computer Icon in Windows Explorer, it is Explorer who updates the information about all connected drives.
The sound also appears when you turn off all shields.
Besides, what is so darn horrible about a very light "grrrr" for about 0,5 seconds...?
Anyway, IMHO changing from 5 to 4.8 for this reason is, politely said, not the wisest of all decisions...