HP Pavilion 6535: Whirring sound from unit when powering off

Hi,
I have an HP Pavilion 6535. When I power down, there is a whirring sound
that rises then falls as the machine turns off. It lasts maybe two seconds
at most. It's not part of an existing noise while the machine is running;
it seems to only begin once the machine powers down.

I work in a pretty normal environment -- no pets, no abnormal amounts of
dust.

Thank you!