Cracking noises appeared after 3 days of printing  

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Sounds perfectly normal

Not hearing anything abnormal.

Crackling can indicate poor adhesion or a loosely supported part

Posted by: @dudendy

[...] After 4 and a half hour some weird cracking noises appeared.

It's always hard to judge audio recorded with auto-leveling and other features. I find that "cracking" noises that correspond to nozzle movement are often associated with something coming loose. Typically, it's a hint that your bed adhesion may be a problem. If it happens a long way into the printing of a large part, you might have warping problems. Check the bottom of your print to verify the first layer is sticking evenly.

It can also indicate a part is fragile and could use support.

Observe the nozzle as it moves and see if you see the printed parts moving when the crackling occurs.

Some infill patterns sound crackly too

I'm with bob on some crackling can be blobs or parts of your print hitting either the print head, accumulated filament below the print head, or the fan shroud (a thin part that easily magnifies the sound).  Crackling sounds can also be caused by high infill patterns such as 30% hexagon or grid (even prusa's literature warns you that grid can get loud)

Cracking noises appeared after 3 days of printing

I think your printing speed is a little bit too high. I found it not worth to throw away too many prints because I wanted to save on time. I tend to print slower, especially grid infill pattern as grid does 'cross' into each other. This tends to shear the flow of filament off from the nozzle for a few split seconds and before it has reestablished, the nozzle is already 1or 2 mm further down the line, leaving a nice gap behind every grid infill 'crossing'.

Try using 40 mm/s for infill and perimeters and see how it goes.

Double check that your first layer is sticking well and that your print sheet is clean.

Cheerio,