Could the "scratching" sound be coming from the disk drive as it moves the heads around for reading and writing? A certain amount of that is normal but some problems can cause greatly increased disk drive activity or unusual sounds:

-- On my last computer, an older model with not really enough RAM, whenever I'd run low on memory the system would start frantically juggling RAM by swapping out chunks of data to the hard drive and reading them back in as they were needed. I could tell when this was happening because the drive would be chattering constantly and everything would take much longer than normal.

-- Sometimes when a drive is beginning to fail, the system detects reading or writing errors and retries the operation several times. In that case too, the drive would show increased activity, and reading and writing data would take longer. As I understand it, if the drive detects errors it automatically retries a certain number of times such as 10. If it can't get something satisfactorily read or written in that number of tries, it gives up and returns an error message (which Windows would almost certainly warn you of).

-- Sometimes when a disk drive has a mechanical problem and is on its way out, it actually sounds different just before it finally fails.

The computer I'm currently using is pretty much silent except for the cooling fans. It's also a "tower" and located a few feet from where I sit, so I hear it less than I did my old "desktop" right in front of me. The only way I can tell when the hard drive is doing something is by watching the little green light. The sound system does occasionally click, buzz or chatter in response to someone using a cell phone nearby or my downstairs neighbor turning on her ceiling fan. None of these sounds are associated with slowdowns, though.