I hung a shelf in my bathroom and ever since when I turn the water on the pipe squeaks in the wall. I discovered that the plumbing vent is in the wall behind my shelf. Could I have punctured the pipe with my screw? If so could this have caused the squeak? It did not squeak before. Hate to have to call a plumber and tear the wall out. Any answere?

By screwing into the vent you may have displaced the vent and it touches a water supply line. If you do not see any water, you probably didn't hit a supply pipe. I'd remove the screw, enlarge the wall hole slightly and see if you can see the hole. Try to put some caulk into the vent pipe hole and spackle the wall hole. No need to call a plumber unless you are not confident in doing the repair. I demoed a wall a couple of weeks ago and found 4 drywall screws were in the ABS DWV pipe. Epoxied a patch and no problem.