Noisy PC help.

[Steecy]

I bought an Acer Aspire M5201 PC a while back and it was silent but recently when it starts doing something like running AVG or downloading something it gets very noisy ( sometimes like a helicopter taking off ).
Would this be a noisy fan or something more serious?
I`ve not got a lot of PC building experience so keep it simple please.
Thanks Steecy.

[PhilEnfield]

Check your fans first and give them a good clean .... that's the most likely cause