Hi,
The fans spin at high speed when you switch ON the server, its because the fans are checked for their functionality.
If any fans does not spin or is slow then its reported at POST.
Refer to this document:
https://h20565.www2.hp.com/portal/site/hpsc/template.PAGE/public/kb/docDisplay/?docId=mmr_kc-0100554
Thank You!
I am a HP employee.