Since you haven't found any water damage, I suspect you may be hearing water inside the pipes. PVC/ABS do not absorb sound well. If you do have a leak, it's impossible to tell you how specifically how to find it because every situtation is different. You will just have to look everywhere the drain can be seen, punch small holes to peek inside, and listen carefully to pinpoint the leak. As far a some magical method of working inside a wall without opening, sorry, there is nothing. In the first place, if there is a leak, the water has damaged the drywall and that will have to be replaced. Secondly, the only way to repair PVC/ABS is to cut and install new fittings. You can't trust the quick-fixes sold in the box stores, but even to use them you would have to open the wall.