I'd use the soapy water in a spray bottle trick to find the leak then seal it. If you can't find it, Slime does wonders. I used Slime in several tires and they never leaked again.

Tire shops don't care for it as they say it's mess when they take the old tire off later on down the road, but I don't care. If I can seal it with a $10 bottle of Slime, then I will.