This is a problem that seems to be plaguing many people. It's really annoying, and sounds like I have an entire RAID of spinning disks chirping away in the laptop. While it seems to happen most when i'm doing more resource intensive tasks, I'm hearing this occur even when opening Chrome. The issue occurs whether its plugged to AC or not, but certainly when charging its even worse.
d111 what did they say? I'm having the same issue. I've read about updating drivers and stuff but nothing really worked. Also I just got the latest bios update and nothing. I'm thinking about contacting Dell too.