Squeeks at login

Hi all,
I have a bizarre experience:
I rebuilt my son's PC from a box with high fannoices to a new where I could fit a decent CPU-fan among other fans.
Everything went OK, except the usual cursing, and the PC booted and ran like a charm and QUIET!
About 48 hours later my son comes and says there is a noice during startup. And YES!!! After post just as the login screen appears there is this chirping sound in the internal speaker. Login and the sound disappears after a few seconds and the PC runs as nicely as ever. Virusscan - nothing, Spyware - nothing, no odd processes.
Tried deinstall/reinstall of Nvidia driver, a few reboots, quiet.
Restart after a few hours, chirping again.
Tried repair of XP, a few reboots, quiet.
Restart after a few hours, chirping again.

Regards and thanks for any suggestions
Hans
Config:
Asus P4T533
512 Mb Memory
Asus V8460
P4 2.8 GHz
XP Pro (all SP's and patches)
Plextor 708A/UF
Yamaha CDW/SCSI
120 Gb Harddisk

 

Hi,
just thought I'll let you listen to the squeeks.
I'm, beginning to think we have some kind of heat/fanspeed related problem. Reboots seem to work OK, but when the comp has been switched off for a while (and thus cooled down and fan probably running at low speed at boot time) the squeaking starts.
Regards
Hans