Insufficient filament output

[holgrich]

I just started to print my first really large object and had huge problems with the filament output. As you can see on the pictures, the filament seems to have been outputed fine in the beginning, but after a certain time the print becomes kinda holey. Further upwards it gets better, but then it gets really bad.
Sometimes I can hear a loud crackling and I think that it comes from the extruder, but I don't know if it just doesn't hold the filament tight enough or if something is blocking the filament on the way to the nozzle/heatend.
My idea was to remove the hotend and clean it (and/or the nozzle) but that seems to be quite an effort and I was hoping that you might have another idea in the first place.

Anycubic i3 Mega
Red PLA Filament 1.75mm @ 200?C
0.4 mm nozzle
Glassbed 70?C