I saw a real cool DVD from Petty called 'Live in Chicago' or something. One thing I noticed is that he really has his mic technique down. He sometimes changes the angle so that his mouth seems to be 'under' the mic and it looks and sounds like the sound picked up by the mic is mainly coming from his nose.
On his records you often hear him change his vocal timbre where it almost sounds like a different singer. I think that's really what I heard and saw on that DVD.
Also, it often sounds like there's some kind of mild flanging going on and I wouldn't be surprised if that's also due to vocal technique.
Andi
www.doorknocker.ch