Ever since I picked my GM up I've heard what sounds like a vacuum leak under the dash. While looking for the leak this last weekend I found it is coming from a white plastic piece wrapping half way around the shifter shaft. I have not pulled the part yet to see if it is cracked or something. The hoses connecting to it do not seem to be leaking and the sound seems to be coming directly from the plastic piece. The sound gets quieter when when I press the accelerator. The car idles fine and seems to shift fine. I cannot image that this thing is supposed to sound like it is leaking but I would hate to replace it to have the new one do the same thing. I question if it is actually bad since it does not appear to be causing any problems.