i've been getting the p0430 code (catalyst inefficiency, below threshold bank 2) error for about a year and never got around to fixing it.
but just a couple days ago, i started getting a sound from the engine bay after i turn the engine off. it does not stop and it is not the fan. it sounds like a small electrical motor running and if i disconnect a small brown wiring harness, it stops. can't tell what the wires run to, but the car drives just fine with it disconnected. one end comes up from underneath the engine toward the left side of the intake scoop and the other end emerges from another bundle of wires and drops down to meet it. once i pointed it out to a mechanic, he immediately said it looks like an o2 sensor lead. but i didn't know they actually made noise.
knowing the code i had before, i wonder if it is the o2 sensor if there's a connection. hopefully i can take care of both issues together.

o2 sensor = noise???? anyone?

 

does your knock sensor make noise? does your mass air flow sensor make noise? i dont quite think so. post pics and you may be able to get some more help. or ... go out and buy a haynes manual.

 

An O2 sensor won't/can't make noise. It is, as the name implies just a sensor - it has no moving parts.

 

Just a guess, but could it be the electronic motor mounts? Seem to remember that when they go bad they make noise when the car is off.