Want to make it quite? Switch it off, open up, remove the media and bin it (the filter, not the media) and replace with any decent fliter, like a Tetratec or Eheim....
On a more serious note, as Fluvals get old, they get noisy and then loose flow, usualy in that order. After this they then tend to start leaking
If it is air, it will self purge within a few hours if just left. Is it like a trickling or a propper grinding clicking sound? There is also the possibility of one of the seals having partially failed... If this is the case, leaving the filter to stand switched off will make the offending seal seap water, making the issue obvious.
If it is age, replace the impeller, bearing, impeller cover and motor head. The cost will be as much as a replacement filter, so if age and resulting wear and tear is the fault, just replace the unit, preferably with the afore montioned decent brands above
A single Eheim from the Pro or classic range will last as long as literaly six Fluvals from the 04 series, and indications from customers at my place of work surgest that the 05 series may be just as bad... You will get 30 years from an Eheim before upgrading it, dieing or loosing interest in the hobby... Count yourself lucky if your Fluval does 5 years...
I have had the 304's bigger sister, the 404, shock me due to voltage leaking from the motor unit into the tank. A mate of mine has had one dump 50l onto the floor in about 2 mins, the time for him to run from the dining table to the tank across the room and shut the cut-off valves off to stem the flow of water
My personal method of priming the 404 was to syphen a bucket of water through the unit before re-attaching the outlet and starting it up. In the likely event of an impeller jam, I applied a hard kick, to where the MSF badge is...
All the best
Rabbut