Whistling/hissing sound when in Park or Reverse ONLY. 2003 4.4

[wullaby]

You know I have 2003 4.4i with 80K and i noticed some hissing noise, so I brought it in to a shop and they lifted it and we both looked, he says he doesn't see too many BMW's (The shop was Goodyear) but he said not to worry and keep driving it, but the noise came from above the transmission pan (the spot where you would drain and fill your trans fluid) the noise was like steam being released, not terribly loud but noticeable. There was a slight leak of tranny fluid and he tightened something and said that should help the leak. I wonder if I have low trans fluid that may cause it, he said not to worry and keep driving if it was him....I want to get a second opinion as well.