Replacement fans for Dell Inspiron 3847 Desktop

Hi Everyone,

I have a Dell Inspiron 3847 desktop purchased January 2015,

It used to run really quietly, but recently it's sounding louder during operation,

I've had the side panel off a number of times, but couldn't pin point what exactly was responsible, but once the side panel is replaced, and the PC is placed back in it's usual location, the sound is there and it sounds like one of the fans,

It had been suggested to me, to place the PC on a softer surface, as it could be just down to resonating. I've stood it directly on my floor in my office, but the sound is still there, a varied hummmmmmm sound which on further listening, seems to be the chassis fan,

My question is, where can i purchase a new replacement chassis fan (the one on the back)?, also , if it turns out to be the CPU fan, can I replace just the fan without disturbing the heatsink, thermal paste and processor?,

I also need advice where I can purchase a replacement CPU fan?, it's an Intel core i5 4460, 4th Gen,

Any help would be greatly appreciated,

Thank You,

Mr Anthony