My son makes the sweetest little sound of pure contentment when he's nursing--he does this little half-moan, half-sigh thing while holding tightly my thumb with one hand and my forefinger with the other. He does it rhythmically while he's breathing-- on the out breath, he makes a very soft "Mmmmm." It just makes me melt!
Also, he has always latched on with complete voracity--moving his head back and forth excitedly and making a loud breathing sound while I get the breast out and ready, earning him the nickname "The Piranha." I had always looked forward to breastfeeding and thought it would be wonderful, but it has far surpassed my expectations!