My MacBook fan was roaring. Again.
This time, however, activity monitor didn't show much going on. It wasn't a Flash ad running in another user account. I didn't have a print job stuck in the bowels of Apple's dysfunctional printing framework.
It had been doing it for weeks. Sluggish performance, slow fan pinup, then continuous running. Something was draining performance and making heat -- and it wasn't showing up in Activity Monitor.
Or, at least, it wasn't obvious in Activity Monitor. I did see something called fsck_hfs using up 10% or so CPU.
To make a long story short - the problem was a defective Time Capsule backup. My MacBook was running fsck_hfs, a utility that "verifies and repairs standard HFS and HFS+ file systems". When I saw this I thought there was something wrong with my system drive - but it tested out fine.
A SuperUser tip clued me in. My MacBook was trying to verify the integrity of my 150GB backup -- over a WiFi connection. This is a singularly ineffective strategy, it would have taken days to complete. The laptop never ran continuously for that long; and there is something about this process that makes a MacBook run hot 
If I'd known what to look  for I'd have seen something like this:
Except it would have shown 2-4%. It only got to 92% when I connected the laptop to my TC via wired ethernet. It got to 92% then stuck there. The backup was toast.
I then disconnected all users and tried deleting the .img file for my MacBook from TC. That failed because the Finder can't handle TB scale image files . I then used AirPort Utility's very confusing Time Capsule UI to erase the entire drive and started the slow, painful, recreation of my backups.
My MacBook is quiet and responsive again, and fsck_hfs free.
That was painful.
Backup is an unsolved problem.
- My MacBook fan was roaring. Why? 5/2008.
- Obnoxious old Epson Scan bug: EPSON Scan cannot be started 2/2008
- OS X 10.5.5: CPU pegging with Firefox 9/2008.
- When Safari locks up - Kill Flash process
- iMac G5: fans running more often 10/2006
- osx - Why has my Mac been running fsck_hfs for two days now? - Super User - great discussion of this problem
- Time Machine. Backup failed with error...: Apple Support Communities 9/2008
Attempting to mount network destination using URL: afp://;AUTH=No%20User%20Authent@Molly.local/Molly_InternalBackup failed with error: 21Error writing Time Machine Information file: /Volumes/Molly_Internal/Stanford_MacBook_0017f2f04828.sparsebundle/com.apple.TimeMachine.MachineID.plistError writing to backup log. NSFileHandleOperationException:*** -[NSConcreteFileHandle writeData:]: Input/output errorEvent store UUIDs don't match for volume: Escanaba
Jul 20 18:27:55 localhost com.apple.launchd (com.apple.SystemStarter): Failed to count the number of files in "/System/Library/StartupItems": No such file or directory
cd /System/Librarysudo mkdir StartupItems