...most probably meant Power Supply Unit...
Might be your fans...and if so you'll know it's coming from them. However, the system should lower the speed of your fans if it's cool enough.
Do you know what your temperatures are? You might be able to check in the BIOS. However, the AMD Athlon XP processors don't have onboard temperature sensors on the chip so it depends on whether your motherboard has one near the socket.
Anways...you can download SpeedFan from http://www.almico.com/speedfan.php
and if it can detect any sensors on your motherboard, then it will give you temperatures of the CPU, Hard Drives, overall system, as well as tell you how many Revolutions Per Minute your fans are rotating at.
If you find the temperature and RPM numbers, either in your BIOS or using SpeedFan, post them here.
Oh...and perhaps I should mention....you can control the speed of your fans using SpeedFan. However, the settings will only apply when the application is running. Although it may be a short term solution, I wouldn't recommend it because it won't fix what's causing the problem.