Conventional questions as you did not mention it:
did you check the fluid level?
does it happen ALL the time
does it happen only when engine is at low RPM and you turn the steering fast?
Test the two last questions and report. Power steering pumps DO have a limited flow capacity and at low RPM they may deliver less than the steering requires if it is turned fast. ALso if fliud level is low, it can suck some air at a time the fluid did not have time to return to the reservoir.
Beside all of that.... it can also be as simple as a loose power steering belt....