The fan has a speed control. It will spin up the fan to the maximum speed as it finds something attached to the expansion bay. (Network adapter) Then will look for the HDD. If it doesn't find the HDD it will slow down the fan again.
I guess it does that because the HDD draws power from the PS2 and Sony engineer wanted to be sure that the PSU will be aways running cool...
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)