What colour is the fluid - as in is it 'dirty'?
If the fluid is a kind of dirty black (including the froth!) you might have a problem with the seals in the steering rack.
For me though the foaming is the key thing, it could be that it's being caused by the power steering pump drawing air past a worn seal or the rack itself drawing air through a knackered or leaking seal as the car changes direction.
Personally I'd top the fluid level up to max and with the car running get underneath it and have a look at the condition of all the rubbers etc and look for leaks around all the steering pipes and the rack itself. Carefully/gently have someone move the wheel from lock to lock and make a note of when the noise(s) are at their worst, it'll give you an inidication of where you need to look hardest.
Start with plenty of fluid.....