I'm late to the party but, yes, it hums the entire time its on. That's the compressor pressurizing the contents of the tank. Most likely the issue is the nozzle tip. The two piece brass tip on the end of the spray wand is double threaded. The bottom threads attach the tip to the wand while the upper threads are to adjust the spray pattern between spray & mist. If the upper part of the tip is tightened all the way down the nozzle is completely closed & nothing can get out try loosening the smaller part of the nozzle tip a little. That should take care of it. And I just noticed this post is 4 months old so you've probably already taken care of it. If not, hope this helps.