4" PVC pipe in basment leaks

I have a leak in my basement on a 4" pvc pipe that I believe is a radon vent or a plumbing vent stack (see pics attached) the leak drips down the pipe and drips at the elbow in the picture. This pipe seems to continue through the 1st and 2nd floor, into the attic and through the roof.
I initially thought that I may have a roof leak and water was dripping down the pipe, but when I inspected the pipe in the attic I found no evidence of leakage. The exterior of the pipe was dry. I also went on the roof and found no cracks in the boot around the pipe.
Does anyone else have another plausible reason for this leak?
Id appreciate any thoughts on my predicament. Thanks
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responding to http://www.homeownershub.com/plumbing/4-PVC-pipe-in-basment-leaks-5763-.htm homerep4 wrote:
gmdkrumm wrote:

Hello,
I do not see the pics to get a better idea, but to me sounds like either its leaking from the fitting tieing a branch line into the main stack or leaking on a branch line close to the stack. The only other thing I can think it might be if its not leaking from the roof would be a water supply line leaking onto the drain stack and running down.
------------------------------------- Arlo Perry www.homerepairsdoctor.com
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