Matt,
Porpoising - If it got better when you went to three-under, and you hadn't changed your nockpoint, then you were probably too high in the first place. I say this because when switching from split to three-under it is normal to need to raise the nockpoint. Another thing to try is putting TWO nocksets on the string, one above and one below the arrow. This prevents the arrow from sliding on the string.
Noise - I think you already have a handle on this one. Heavier arrows soak up more bow energy and result in less noise. Another way to think of it is that any energy left over after the arrow leaves the string has to go somewhere and it will be reabsorbed by the bow in the form of vibration. Vibration makes noise. It also makes handshock. Something that has already been mentioned is the tightness of the arrow nock on the string. An arrow should stay put if you put an arrow on the string and allow it to hang down. A light tap with a fingertip should make it fall off.
John