Sounding as though it's right under your feet, I'd say the catalytic converter. I had one go bad on a Silverado that about drove me batty. I took a rubber mallet and lighted hammered the exhaust and as soon as I tapped on the converter...bingo.
The IWE''s (inner hub) can go bad and give an unpleasant sound. I helped a friend fix one not too long ago. We were suspecting a vacuum leak but it was just one of the seals failed on an IWE.
