Line level Audio Input low on MacBook Pro

In comparing my mid-2009 MacBook Pro with a mid-2008 MacBook Pro, the audio level that gets into recording sofware via the Line-In jack is very low. With the same audio being recorded, via "Y-adapter", into both machines at the same time the level into the mid-2009 is about 1/2 the level into the mid-2008 machine. Both are recording with the same software (Audacity), setup identically, there is a marked difference in the two. Is this a known difference in hardware or is there some other explanation. I would appreciate any input as I have spent quite a bit of time trying everything I can think of that could possibly affect this issue. Thanks for any response.