My Avast appears to be in the middle of a huge system scan for no reason. I don’t see anything in the UI that suggests it’s busy, but it’s using a little cpu time and a lot of disk activity, and it’s launching the sandbox engine. According to Process Monitor, it’s accessing files in sequence across my huge data drive, so it looks to me like it’s scanning, but i would really like it to stop! The file monitor status screen says it’s not doing anything, so i don’t think it’s on-access scanning going on.
A (very slow) reboot stopped it, but i still don’t see any acknowledgement in the program or the logs that i’m aware of that it happened. 
Spooky.
Now, it might matter that avast! had downloaded and installed a program update but had been unable to update all of its files, so it wanted me to reboot. I told it no, i’d do it later, and after a couple of days of that it stopped asking me. The UI said that it was on the new version, although i believe from past experience that the driver, at least, had to be the old one. Now it’s running the newest everything, of course. Can’t wait for all those BSODs i hear about! 
Do you maybe use Raxco PerfectDisk 12 ? Also what exactly suggests to you that it was avast! that caused the HDD grinding? It could be some other program triggering all the files on disk and avast! just started scanning them because they were being accessed (and it’s avast!'s job to check them).
I’m not using PerfectDisk, no.
The reason i suspect Avast was doing it on its own was that Process Explorer showed large amounts of I/O and disk activity in the AvastSvc.exe process, but basically none in any other process. I think it’s unlikely that another program was accessing the files because it should have shown up as I/O or disk activity if they were. I can’t say it’s 100% certain, though.
Did you maybe look at the File System Shield? Was there any activity on the graph?
I did look at each shield graph, and none were showing any significant activity. The graphs were working, because the file shield graph did show a couple of blips here and there when i launched something.
As long as it doesn’t do it again, i’m content to chalk this oddity up to the partial-update state i mentioned in the second post.
If you want more information, i’ll be happy to answer. For one thing, i’m running Windows 7 x64, so i don’t expect it’s the same PerfectDisk/Ghost/Acronis issue as the others are having; in fact, i don’t have any unusual, disk-management programs on here anyway, though Windows itself likes to defrag once in a while (but i didn’t see any mention in the event log of it starting a defrag session at that time).