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My gas light was on this morning so I went to fill up as usual. I drive out of the gas station to a light....and I notice the car is idling very rough.  I went back out in the day...same thing rough idle...and it seems to be worse when the A/C is on....and I can hear this clangy metal sound.

Did I get a bad tank of gas? Would that cause the car to idle rough? Car was fine yesterday.

To provide some additional info:

The rpm's are in the normal idle range. I listened to it again tonight and the roughness of the idle seems to "pulse" of sorts....but again the rpms stay steady.  I think the "clanging sound" is just something metal vibrating against something in the engine bay due to the rough idle.

My friend said I should check for vacuum leaks....also said I may have gotten some gas with water in it.

It is possiable that you had some stuff in the bottom of your tank that got stirred up when you were refueling. Although without hearing it, it could be anything.

I drove the car today and emptied about 1/2 of the tank.  I filled her up with some 93 from a shell station (my original fill was from chevron). I drove the car around to get the new gas mixed in and it still has this strange idle rattle. Only happens with car in gear sitting still. Goes away when I accelerate.  Its pretty loud...makes the car sound like a beater at a stop light. The car drives fine though....I drove 70 miles on the interstate today...no issues. I wonder if something got knocked loose.

Figured out the problem tonight.  My cat converter is vibrating like a bitch.  I have the DC sports 4-2-1 header on the car....I am guessing the flex-pipe has torn like many others...or a bolt is loose.  I don't have any of my tools with me when I'm in school....suppose I will take it to a muffler shop.

Update: went to the exhaust shop thinking the flex pipe was broken. They put my car on the lift and we took a look. The flexpipe was fine....the cat-converter was shot...the internals were rattling around. They said it looked like the internals had started to melt....strange.  To make a long story short, I had the cat converter replaced with a Magnaflow High-flow CC for $350 including labor...and they did the work in 45 min. Rattle is gone...car seems a little Peppier!

Sweet! Glad to see you had a positive outcome!

Good shit. How many miles are on your car. It's not too often you hear about somebody's cat goin especially melting the internals!

The car only has 75k miles on it.  Looking back on it I wish I had examined the heat shield on the CC a little more closely.  The guy smacked it and I heard it rattle...but looking back on it the heat shield may have just been loose.  But this guy has been doing custom exhausts for many years...and I trusted his opinion.  When I called on the phone thinking my flex-pipe had separated his first response was "well, I might be able to just weld it back on"....so I don't think he was just trying to sell me something.  Either way the horrid rattle is gone and I have a high-flow cat now :)

hm. cat is part of emission system so it should be covered under emission warranty from honda i think

You would think so.

I checked my warranty booklet and there is a section that says "Things not covered". Exhaust and catalytic converter are the first two things listed.

Clapton is correct.  I can vouch for catalysts not being covered under extended warranty.

looks like i might be experiencing the same problem.

I have a borla catback exhaust with approx. 60K on it.  About 2 weeks ago I starting hearing a metallic (tin like) rattling upon acceleration.  Sounds like something is loose when driving about 1500-2500 RPM's.  Been pissing me off for a while now.

Finally got around to taking a closer look and the noise seems to be coming from right about where the cat is.  Since it is wrapped in a thin heat shield, I can almost guarantee that is what the problem is.

Yeah, take a look at the heatshield....might just be loose.  I wish I had checked mine before I had my entire cat replaced. I took the shops word that my cat was blown.

I might have the same problem but I get this vibration or rattling when I cruise at 45 + mph but when I'm sitting, it doesn't make any sounds. I hope it's the same as yours.

Does your car bog down at all?

Silver bullet, it probably is just a heat shield around the cat. I had mine rattling after my shop torched the bolts off to install my header. Ended up loosening the upper shield around the cat. So I had that shit ripped out. I would suggest ripping off the shields b4 you go replacing the cat.

@crackerteg, what do you mean by bog down?

@fabulous010, I hope that's the case. I did run over a parking bump by accident and i bottomed out not too long ago so that could possibly be why...

From the freedictionary.com:

1.   bog down - get stuck while doing something; "She bogged down many times while she wrote her dissertation"
bog
break off, discontinue, stop, break - prevent completion; "stop the project"; "break off the negotiations"

2.   bog down - cause to get stuck as if in a mire; "The mud mired our cart"
mire
get stuck, grind to a halt, mire, bog down - be unable to move further; "The car bogged down in the sand"

3.   bog down - be unable to move further; "The car bogged down in the sand"
get stuck, grind to a halt, mire
stand still - remain in place; hold still; remain fixed or immobile; "Traffic stood still when the funeral procession passed by"
bog down, mire - cause to get stuck as if in a mire; "The mud mired our cart"

4.   bog down - cause to slow down or get stuck; "The vote would bog down the house"
bog
slow up, slow, slow down - cause to proceed more slowly; "The illness slowed him down"

Nah my car never bogs down. When I cruise at 45+ there's just a rattling sound coming from underneath the hood.

Probably a heat shield like Fab said.

Resurrected.

Finally got to the bottom of the rattle.  First thing i did was check the heat shield - all seemed fine.  Bolts were tight and not loose.  There is a hanger right in front of the cat though with a rubber mount that seems to hold the cat up.  the rubber went bad and dropped the cat an inch or so which caused it to rattle against the metal bracked instead of being held in place of the rubber.

in a nutshell - easy fix.  glad it wasnt the cat.