Duct work issue

I have a 3 year old 2 story home w/ flex duct through out. It uses only one 5 ton A/C unit to cool both the upstairs and downstairs. I know I'm creating static pressure behind the air handler, but I have to close the vents downstairs in order to get the air upstairs. I ussually hear hissing / whistling from the vents downstairs. Today the A/C has been running constantly and I don't hear the hissing / whistling coming from the vents. What just happened? Obviously the thermostat is not turning off b/c of the lack of cooling. I believe I created a leak in the ductwork. It's a cheap house and I've been told that builders often don't install the ducts correctly. I cannot fully access the attic where the ducts come from downstairs. I went around looking / feeling for leaks, but lack the tools and know how to do so. How can I inexpensively determine my suspicion?