[timinky]

Been reading a few threads here and got me wondering too if my unit
has began to "hum". Only had this thing for a month now. There is a
faint audible hum when the iMac is on. No vibrating or anything like that.
Not loud like a flourescent light ballast or anything. Just a faint low audible
hum. iStat says fans are running at 1198, 1198 and 998. I guess these are
normal ranges. As I type 90-95% of CPU is idle. Guess I'm just trying to
determine if I have a "humming" iMac as other threads have alluded to.
In reality is this thing suppose to be whisper quiet. Should the fans run
non stop? Did the SMC reset & didn't notice any significant change.
Of course I only hear this "hum" when sitting quiet in my office, no other
noise to dampen it. In the case when other things are creating noise, i.e.
TV, sound system, etc., sure it's there but drowned out for the most part.

[Veltliner]

Someone in another thread described the problem as a bad LCD inverter.

It's a unit that creates high voltage for the LCD.

Test for this this way: reduce your screen brightness. If it is getting louder, than it's very likely the problem.