Does sound like a speaker problem, but... the last couple of times I heard something like that it turned out to be the heating duct above the speaker, not the speaker itself.
Disconnecting the other speakers (not turning them off in the receiver) usually helps to be sure it's the speaker acting up. The other option would be to swap wires between the VP150 and a main (probably better than swapping with a surround) to be 100% sure.
Definitely go ahead and talk to Axiom anyways, this is just something to do if you're bored
