Assuming your sub cable is decent and not vaccum abused:
Any recent changes in back of your reciever? Cables shorting perhaps? HDMI and other connections ok? Definitely seems like a stray signal or filtering issue somewhere in the chain.
I had a similar issue develop over time using an extended sub cable joined with an rca butt connector. Got grumpy with time.
I would do a hard shutdown (unplug your receiver) and reterminate if issue persists. Failing that, call axiom.
Best of luck.
Edit: can you run a cable direclty from your bluray/dvd/cdplayer to the sub to rule out a huge chunk of the signal chain. Even an ipod on batteries connected directly would be a good elimination test. Make you turn the sub volume off first and slowly turn up if you connect a device with no variable output. Really loud!