2005 Ford Expedition Engine oil keeps going low, but no lea

I have a 2005 Ford Expedition - Eddie Bauer Edition - 5.4 Liter engine. I have recently noticed that the engine oil keeps going low by about 1 1/2 qts, and they're no leaks to be found. I have searched everywhere! How I noticed the problem was when I drove the vehicle I noticed the lifters sounded very loud. This had never happened before so I thought nothing of it, until it happened all the time. I decided to check the oil level and found it was 1 1/2 qts low. How could this happen? I added the amount that was low and everything seemed fine, but I just recently got another oil change and it has happened again. I have never heard of this happening and I am a long time ford owner. Could it be a defective PCV Valve that allows the oil to pass through? Please help!