[Jet-Lee]

1) I don't know
2) It may just be vibrating from the vibrations created by the subwoofer next to it, esp. if they have a shared volume.
3) different tones produce different rattles, as well as volumes, even differing excursions. Say your playing a 50Hz test tone on your old subs at 1/2 volume. Maybe that creates a decible reading of 110db (hypothetically). then you play your new subs at the same tone, volume down to get the same decible. If they move more, they will displace more air which can make more things rattle than the old system.

Make sense?