I'm guessing, yes, but I'll ask: have you changed all the o-rings? Also, my kegs had plastic gas dip tubes that smelled like soda. I replaced those with new ones (Williams Brewing sells them - standard stainless won't fit.)
Yep replaced all o rings and cleaned inside the tubes with a brush. Soaked overnight with baking soda and hot water initially and oxy clean prior to using. I have 4 but this one is just stubborn.
You could also try some 5-star saniclean. I bought a bottle once thinking it was star san. Works pretty good knocking the odor down in fermentor buckets. Might help with your keg too.