Acers Aspire 5720 fan noise or fan is always on

I bought this PC new about 6 months ago, I've already returned it once as it
was not charging and they replaced some AC connector or something.
I've noticed that the fan seems to be almost on continuously even if the
machine is not under load, say just typing a mail as I'm doing now when it's
been started up from cold (i.a. not been in use for hours). CPU usage is
sitting a about 1 - 4 % and I don't have any disk activity or network
traffic.
I noticed that if I'm say on the BBC News web site and watching a video
clip, the cup usage will jump to 100% and the video becomes unwatchable with
jerky video performance and the sound is broken up.

I noticed the same if I try to play videos or sound from the local co drive,
it's unusable if I try to network stream anything.

I have called Acer and they've given me a ticket to return it on possibly
for a repair but I just wanted to know if anyone had a solution or
experienced similar issues.

Thanks in advance..