<p>My 3D printer makes weird sounds. When it's at >75% printing speed the extruder motor makes a "tac tac" sound and it goes backwards, pushing the filament back, for a small interval of time. I have tried changing the nozzle temperature and I'm unable to work this out alone. </p>

<p>Has someone had the same problem?</p>

<p>This is the 3D printer: <a href="http://www.dx.com/es/p/geeetech-high-quality-wood-geeetech-prusa-i3-pro-w-3d-printer-kit-469707?tc=EUR" rel="noreferrer">Geeetech High Quality Wood Geeetech Prusa I3 Pro W 3D Printer Kit</a>.</p>

You are extruding (rather, attempting to extrude) faster than the hotend/nozzle can melt & pump plastic. Eventually something's got to give, and it's usually the grip on the filament by the extruder gear (or the torque available is exceeded.)
You either need to limit the speed you extrude at, or change other print settings (temperature) to melt faster. You are not going 75% - you're trying to go more than 100% (in real terms of what the printer can actually do.)