Noisy computer

[dmcd]

my computer sounds noisy (humming) - I have taken the back off and sprayed the fan with air but it is still loud. What can I do next? I am a beginner so please keep it simple. What could it be? My computer has also been running slower than usual - is it related??
thanks

 

[ih8tmycmptr]

what are the specs of your machine?
has it always been "noisy(humming)'' or is it a new thing?