Car making wierd noises

I have a 99 civic ex auto. Anyways, i've had it for almost a year and today i heard some very wierd noises. After i came home from the uni, i started my car to go grocery shopping and whenever i would take my foot off of the accelerator, it would make really weird noises as if it was grinding on something. Then i put it in park, reved it a few times, and the same problem occured. I have driven it 200 miles over the service mileage. Is it serious or something minor? Oh yeah, the noise seemed to be coming from the center of the engine. This all started when i turned left. I did quite a few right turn but nothing happened. Also, i have a code 67 for the check engine. I was waiting to get some money before i fixed it.

Code 67:
Catalyst system Efficiency Below Threshold
malfunctioning or defective catalyst system

i don't see that being the cause of the noise though. when noises happen when you are moving or driving it's possible it has something to do with the drivetrain. i don't understand why you're saying it's coming from the inside of the engine.

Have you ever taken a technology class and played with the gears? Have you heard the noise when they are not meshed properly. You know, the tick, tick, tick, tick.... That's how the noise is. I opened the engine and had my dad rev it up, and the noise was coming from it.