I have 90 year old home with cast iron waste pipes. One 2nd floor bath sink gurgles, after cutting into wall behind sink I have cofirmed the obvious... sink isn't vented. The cast iron waste pipe rises 6" above the sink drain with a hub. This hub is filled with a very hard chocky white stuff. Doesn't seem to be sandy, so could this be concrete or some other 1920's pipe filler? Is drilling with concrete bit the only solution?

Don't know where you live so this may not be allowed by your local plumbing code. You could solve the issue by installing a Studor vent or AAV. Has this sink always gurgle every day since you moved in?

I agree with Bob that an AAV under the sink is the best approach. However, if it is you desire or intent to install a proper vent line, I wouldn't try drilling out what ever was used (probably plaster) to seal the old cast. There must be a tee where the sink drain meets the drain pipe. I would try removing that section of pipe at that joint and using PVC to vent. Is that section of pipe cast iron or galvanized iron pipe. If galvanized I would cut it below the sealing material and convert to PVC with a rubber coupling.

A little note.
Codes mandate that if you use a AAV that it must be assessable for service or replacement. In other words you can't seal it up in a wall. Good luck. Tom