I just got another 1220HO. When I hooked it to my 500 watt amp, it too would make a distinct rattle
, but only while really being pumped and only on certain (super) bassy selections of playback (whether movie or music).
I sighed and pulled out the driver and played with it some in free air, which led me to discover that on this one, the spider ring (outer ring the spider is glued to, which is in turn glued to the basket) had become partially unglued. Luckily this is an easy fix. First I broke the ring the rest of the way loose from the basket, then mixed up some high grade 5min epoxy (Gudgeon Bros G5 from West Marine, but I'm sure any would work) and glued it back, being very careful to keep it aligned properly. No more rattle- whatsoever.
The original glue appeared to be a polyeurethane type glue such as Gorilla, which had become brittle with age. 'Normal' playback did not reveal this issue, only really making it work brought it to light.
Again, I think you should pull your driver pump it out of it's enclosure. To do this, run extension wires from the amp to it's spades. Be vary careful not to short the wires together while it is playing heavy- best to electrical tape at least one spade plug to reduce the chance of contact while your strugling not to drop the very heavy driver while your inspecting it. It sounds to me like your spider ring may have also come partially unglued as well.
PS. the most punishing/revealing sub testing cd I have is called 'Antimatter vs. Antimatter' (Asphodel label). Much better than waiting for the rattle to strike at random...