Any ideas why my EP500 sub would be rattling or vibration noise on a less then 50% setting just seems to have started doing it this last month? I thought it was a picture on the wall at first. Anyone have this issue?

Try snugging up the srews on driver surround and amp plate?

Definitely have to isolate the rattle directly to the sub vs. anything else in the room. For the longest time i kept thinking it was our sub (in our old house) and it turned out to be the door on the electrical panel in the corner next to the sub.

If it really is an internal sound, you could unplug the sub, unscrew the back panel plate and take a look at any of the internal screws or wire leads to the driver (assuming you are comfortable with such activities).
Otherwise, beyond that, probably should consult with Axiom folk and possibly need to return it.

Thanks guys I am going to do a bit more investigating and check all the screws before I send it back.

mine from many years ago did the same thing, at certain frequencies and during explosions. Ended up being air escaping around the amp plate. Snugged up the bolts and all gone. It can also be a wire rattling internally. Try removing the driver and inspecting the inside.

What version is it?

Old story but relevant. I had a cat toy stuffed up the port. Thought for sure I had a bad sub till I found it.

Weird thing: He didn't own a cat.

I did see Brian lurking around the house that week though.

Hard to find places to dispose of old cat toys today.