Hissing to me could potentially be vacuum. On my 95 e320 I hear a hissing every so often its the AC running. I'll listen more closely and see if I can't pinpoint it. I just assumed it was a vacuum leak that was "recharging" with additional vacuum once it got too low.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg