External Filter Kicking Out Bubbles!

Hi, I have an "all pond solutions" external filter that has, just recently, started kicking out bubbles... constantly and generally making a noise. The flow of the water seems to have dropped considerably too. There are no leaks but I realise that air must be getting into the system somewhere.

Does anyone have any ideas what may be causing it? I haven't changed my setup at all (except for making some of the holes in my spray bar slightly bigger). Although I have now halfed the amount of holes and the problem is still there. could it just be the age of my filter, 4 years?

Interestingly, the problem seems to disappear if I take the spray bar off and put my finger over the end of the pipe, dramatically reducing the hole and increasing the flow. 

Stuart.