I just bought used Maxima 2002, stick shift with 148K miles. When the car on ideal, the engine runs very smooth no knocking or clicking; however, when I press the gas, whether I am driving the car or parking, rattling sound come from the lower part of the engine. The sound can be clearly heard just after I take off my leg from the gas pedal.

The engine has a lot of power. Not weak or anything like that.
I have changed the motor oil, but the sound didn't go away.

There is "service engine soon" light on, I read the code, and it was P0300 (Generic Misfiring), I am about to change the spark plugs, but I don't think that the problem is from them because the engine runs smooth on ideal.

Any Ideas? What the problem could be?

 

perhaps posting a video via photobucket or similar and posting the like here would help us help you better.

 

Maybe the timing chain rattling?? Worst case could be a rod bearing taking a crap, but unlikely I think.

 

Loose heat shield, maybe? my wife's Outback makes rot of a rattling tin can noise at times, and that's her issue.

 

Does it sound like what the one in this video sounds like around 25 seconds?



If yes, it is definately your pre-catalytic converter. The inside most likely came loose and it is rattling. I'd suggest you stop driving your car right away until you get it replaced. If you keep driving it, the converter material will flow back inside your cylinders, and create a sandpaper like texture on your cylinder walls (if it hasn't already) I drove mine like that for a few months and it destroyed my piston rings. My car started consuming oil as soon as the problem appeared. Now two years down the road, my car is drinking oil like crazy (1 qt per 300 miles)...

If it's not the same sound, disregard my post.

 

To confirm it is the rear precat, and not the front, lift up the front of your car, go underneath and have someone rev to 2k then lift off the gas in neutral.

Put the end of the screwdriver on the exhaust on each side of the engine before they Y together, and put your ear against the other end (handle) of the screwdriver. You will be able to notice where the sound is louder. If it's the front, then replace your front cat converter:

http://www.ebay.com/itm/Maxima-3-5L-...ht_3110wt_1392

If the noise is coming from the rear, replace your rear catalytic converter:

http://www.ebay.com/itm/Infiniti-I35...#ht_700wt_1159

 

I'm going with heat shields. Mine were all broken off and rusted, car made a rattle when letting off the gas. Jack up the car and wiggle some of the heat shields precats and the one for the catalytic convertor itself.

 

That's what I thought mine was, the heatshield. That's until I actually held the old cat converter in my hands and shook it back and forth. The converter material inside it was still in one piece, but came completely loose.

When you lift off the gas, it creates a vacuum inside the exhaust, on the upstream side of the converter, which is the reason why the inside of the converter was being sucked back up, and rattled.

 

Thanks for of the replies.

The sound is almost like Gizm0's video, and it happens after I press the gas. It seems that it is the pre-catalytic converter. However, when I took video today of engine, I heard another noise on the upper part of the engine I am afraid that the converter material flew back inside the cylinders, and created a sandpaper like texture on your cylinder walls as Gizmo said.

I uploaded the video to see what do you think guys?
http://youtu.be/UbDHSTIHorA

 

That sounds like the timing chain hitting the timing cover IMO...but I could be wrong