What's that sound?
 

I came back from a trip and went to take out my 2004 350Z Roadster with 50,000 mi, and noticed a raspy exhaust note that wasn't there before. Drove it anyway and felt a loss in low end power (responsiveness).
Eventually the "check engine" light came on. I took it to two auto parts stores and they both came up with the same P-0300 code (Multiple misfires).
Took it to a mechanic who replaced one of the battery terminals that he said was loose and arcing and cleaned the negative connection, re-set the computer. Car seemed fine but the noise is still there. A day later, the check engine light flickered on and off.
I have to take it back to the mechanic to look deeper into the situation and I'm wondering if anyone has a thought about the engine issue and the odd exhaust note being related.
Thanks everyone!

First easy and quick thing to do is check the spark plugs and also check for a vacuum leak. Spark plugs by themselves can tell how the engine is or isn't running properly. Also by checking the spark plugs you can tell wether you have a valve cover leak in the tube which can cause a miss fire.