Hi,
Have never seen the tank set you have, hence do not know how the filter look like, so I can only offer some general advice.
Generally, the first compartment of a filter should contain filter wool to trap all the waste. The second compartment would contain your bio media (in your case, the ceramic discs), same goes for the last compartment.
Some folks who keep fishes which produce more waste sometimes have a prefilter compartment by having bio-balls in their first compartment, to break down the larger pieces of fish waste, filter wool in the second and bio-media in the end.
Personally, I think there's no need for a prefilter in your case. So just stick to filter, bio media, bio media combination.
I do not know what the flat piece of plastic is... perhaps you can take a few photo of the filter so other readers here can help.
The slurping noise you're hearing is probably bad. My guess is air is being sucked into the filter, in the long run, this will cause the pump motor and/or the impeller to work hard than they should. See if the inlet of your filter is completely submerged, air shouldn't be allowed into the filter.
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