Just to add to the mix…….
I recently bought and set up a 600, l shopped around until l located a unit with the new prefilter pipe and bought it, it also has the new concave head plate.
Initial start up was not good, more noise than the models l’d viewed in operation in a couple of shops.
I spent a few mornings tilting and farting about until all the air was out, same noise level…… Next step was trying a bit of rubber between the venting unit / impeller housing as in this thread, no difference, same noise…. I called Oase Tech Help, their view was the only thing causing the noise would be air in the filter or the impeller rubbing… Next step, remove impeller, get some natural plant cellulose lubricant on the impeller shaft and bearings, result is pretty much silence😃.
Flow rate from my 600 on a 160 liter aquarium is 3.35 tank volume / hour, l’m shocked at how low that is for a quoted 1200 liter / hour pump. I have two trays of blue foam, 2 of seachem matrix, one of sera siporax.