I've been reading the posts about this. For a long time whenever I've flushed my toilet on the second floor, there is a great deal of gurgling that I hear through the wall above the toilet (on the way to the roof vent). My plumber just told me that he thinks there is a crack in the vent pipe going from the sink in the 2nd floor bathroom to the stack and that this is causing the gurgling. How would he know that there is a crack as opposed to a clog in the vent pipe? Obviously, I don't want to open the plaster wall if there's no need. Thanks in advance for your advice.

Get a better plumber. A crack would actually cause less gurgling since a new air source would be added. Its far more likely to be a vent stack starting to clog up. You can try the garden hose but it may just push the clog further down the pipe. Rod with a swer rodder liong enough to reach roof to basement plus 20'

Gurgles indicate that you have a blocked vent and the discharge is attempting to vent through the nearest trap. It's air being pulled through the trap seal that you hear. Cracked pipes may leak but I never heard one gurgle.
Using a garden hose instead of a snake has its drawbacks. While a snake will auger into a clog and break it up a hose depends on head pressure to push the clog on out. If the clog won't budge the water will build uo to lowest fixture upstream from the clog and overflow. You're on the roof and the water's flooding your home. A snake is the only sure to clear a blockage. Good luck, Tom