After a HARD and sustained rain last night, I have a mystery leak. It is about 6' off the floor and appears to run along a drywall seam as the outline of drywall tape is now "puffed out" beneath the latex paint for about 8" and there is a " and there is a " pocket (presumably holding water) about the size of the palm of my hand just below the drywall seam area. One one side is the hall bath (sink & toilet); on the other is the hallway wall. It has appeared on the hallway side (not the bathroom side). The vent pipe to the roof is in this vicinity (located in about the middle of that wall?). In the attic I inspected the vent pipe as well as I could and all appears dry. There does not appear to be any evidence of water that has come in from the roof (poss. Cracked roof seal?)and run down the outside of pipe. I cannot (personally) get onto the steep roof to inspect the roof flashing). Is it possible the vent pipe itself is cracked and if sufficient water from a hard rainfall occurs, the water is running down INSIDE the pipe and then migrates along the drywall seam? The pipe would need to be cracked inside the wall between the attic and the first floor -- does this happen? My problem is at this point --- who do I call (first)? Is this a call to a roofer.. Or a plumber? Any advice is greatly appreciated.

It might be from the rain, leaking at a joint, especially if cast iron and the vent has leaves in it.
Since the drywall needs replacing anyway, why not cut out a neat rectangle for a better view?
If it's truly dry in the attic it sounds more like a plumbing job.

Thanks for your input.. no leaves in the vent... no trees anywhere near it.. new development.. all trees are still 6ft-ish. Attic is dry. Appreciate your opinion my problem is more "plumber" than "roofer" at this point in the process. Will wait for plumber before cutting out any wallboard, but do suspect a peek inside the wall, as you suggest, will be helpful. Thanks!