The flippnat answer is the one that is leaking. I'll read you mind and re ask the question which is the most common t o leak?
There isn't any the IC piping is the biggest culpret buy some t bolt clamps and replace the worm clamps on your IC piping. Teh engine is a maze of vacuum hose and a leak can occur anywhere the leak can be real subtle and hard to find. I recently bought a kit from hose techniques and just replaced all the damn hoses It was well worth the money and looks good to.
I concur with dzeiler. I bought a Hosetechnique silicone vacuum hose kit and replaced all the vacuum hoses. You can search for a hose that's leaking right now and replace it. But down the line another hose might start leaking and you will have to go back and do the leak test all over again. Much easier to replace all the hoses with silicone ones, tighten them properly and never have to worry about them again.