Ive Googled it and came up with different things that cause it.
Noisy power unit, resonation from the back light at certain brightness levels, noisy psu.
But nothing to say whether its a fault or a common thing. My last monitor was a CRT and didn't make a sound at all. So just wondering if this is to be expected from a LCD.
I have contacted Acer to ask if it is a fault.
Be good to know if anyone else has the same symptoms and what can be done about it.