You can pick this one up at Radio Shack. Get Adapt-a-Plug "B". I use it for my 396, BR330T, & 246 (I have two of the adapters; usually take 2 of the three scanners with me when I travel).
So the "ground" side of the transmitter is -12v, and the "ground" side of the scanner is -12v and you're shorting the cold side of the audio output (which is connected to the "ground" of the transmitter) to the ground of the scanner - which is probably what's causing noise. The cold side of the earphone jack is NOT at ground potential. (See all the threads about external power speakers and Unidens.)