Is it pushing the water? There could be air in the filter that will go away in some time or if it is not pushing the water at all or doing it very slowly then maybe your impellor shaft is broken... not to worry as it costs only a couple of £ to replace from a decent fish store.
The water is flowing through the filter and comming out of the spray bar with pressure....i know its not air i know the air trapped sound and its definatly not it, could my impeller still be broken but 'pushing' water still?
Take out the impellor and see but if it is broken, you will not be able to use the filter unless it is replaced as even though its working now it will not work when you re-assemble it back.
get a replacemebt shaft and then open the filter. You dont want to dry your filter out while waiting for the shaft