I don't know Mike's level of expertise, but I suspect an exhaust leak. I know he said it didn't make the noise before, but how closely did he listen to it before he replaced the plugs?
But, as Earl suggested, I'd replace all six before trying to diagnose it.
What prompted the plug change? The OEM NGKs are good for 100k miles. I had mine in for 133k and they were nearly new when I took them out.
