S600 power steering pump question

I have a 1994 S600 coupe, recently, the steering pump started making noise when the wheel is turned, but seems to go away after about 15 minutes of normal driving, the fluid level was low, I topped it off with the correct fluid, but the noise is still there, it sounds like the pump is running with no fluid it it, but has fluid, the rack turns fine, no problems, not tight, the system works fine, but the pump is very loud, any ideas?

air bubbles?

Sleepwalker hi, how do I measure the  power steering fluid level of a ML320 2001 model...?there is a   min. 20oC max. and min. 80oC max....  Thanks, kababayan...

I have the same thing in my 2000 C280. It only makes the noise when turing it to the left and only when it is cold out.