A vacuum leak could cause that I believe. Our 420SEL had a number of vacuum leaks that cause some stumbling on acceleration. Are you sure the hissing sound if coming from power steering system?

Side question, why 20w50 oil? I run 0w40 in my 190E. However its only got <120k miles on the motor.

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k