When I had a rattle problem, I could actualy take my hand and apply a little preasure to the Amp plate when installed and the noise would change. Another thing is that if air is escaping around the drivers or amp, that can make noises. I also noticed that the negative terminal wire post on the back of the amp was loose on mine, so I think that is where the rattle was coming from.
When I played the RealTraps Test tones, my noise was very bad around 20-25 hz, beyond that I could not hear it...
I would just call Axiom, and explain what is going on and what you've tried to fix the problem. They will help you out...
M80s-VP180-QS8s-EP600-2xEP350 Denon3808 Outlaw7700