About a few weeks ago i noticed this like, metallic rattling/ticking (not pinging, i know what that sounds like) when i gave anything more than 1/4 throttle through 2nd and 3rd gear. Didnt think much of it, i saw a few other people on here had the same problem. Other day my CEL comes on, pulled the code and its a P0420, catalyst efficency. Now im thinking that noise might be sh*t rattling inside the converter, however i took maybe 5-6 trips today going back and forth to class and the light went off. So im not sure what to do. I looked through the FAQ's but didnt find too much.

 

nothin huh? p0420 easier than it sounds?

 

I'd start by changing the rear O2 sensor. Here's some text from the FSM:

The ECM monitors the switching frequency ratio of front and rear
heated oxygen sensors.
A warm-up three way catalyst with high oxygen storage capacity
will indicate a low switching frequency of rear heated oxygen sensor.
As oxygen storage capacity decreases, the rear heated oxygen
sensor switching frequency will increase.
When the frequency ratio of front and rear heated oxygen sensors
approaches a specified limit value, the warm-up three way catalyst
malfunction is diagnosed.
Malfunction is detected when warm-up three way catalyst does not
operate properly, warm-up three way catalyst does not have
enough oxygen storage capacity.

Possible Causes

I Warm-up three way catalyst
I Exhaust tube
I Intake air leaks
I Injectors
I Injector leaks
I Spark plug
I Improper ignition timing