I had a "groaning" sound on my 1998 C230. This is a typical symptom of low fluid. I checked the fluid both cold and hot and it was well within the marks. I had the dealer "top it off" resulting that the fluid level ended up being "technically" a hair above the max line, basically when cold the level is at the min line for the hot range if that makes any sense. These marks are pretty small and close together so it is not an exact science. Anyway, long story longer, you might try putting in a little more of the very special magic secret German fluid that no one knows what it is but you have to use it ONLY. Also, there is a filter in that PS pump somewhere. That might be the problem. I am no expert on PS though. Just telling you my experience. Hope that helps.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra