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Vibration and humming, slightly rough idle

25977 Views 15 Replies 5 Participants Last post by  NatDogg
Hey all.
I'm going to start out with an apology. This isn't my car I'm writing about, and I haven't gone over the search outputs with a fine-toothed comb because I'm just looking for some quick advice.

My girlfriend has a 2005 ZX4 SE 2.0L. Recently, it's been making a loud humming/vibration sound when she first starts it. I've heard it, and it sounds like interior trim panels rattling, along with an overall vibration hum. Most of the rattling/humming noise seems to come from the rear. We really haven't investigated it further, because it really only occurs at startup, and (she thinks) only when the engine is cold. Doesn't seem to affect performance at all. The car drives great, shifts smoothly, and doesn't seem to be misfiring at all.

My first inclination is just to change the spark plugs and maybe wires. The car just turned over 70,000 miles this weekend, and I don't think it's probably had plugs done. She's not sure.

Does this sound like a good place to start? I was just going to pick up a set of the copper motorcraft plugs. As for wires, I have no idea. We want to go inexpensive, but not worthless. Any ideas?

This is just her daily driver, and she's not interested in any upgrades. She probably wouldn't even worry about the vibration noise at startup, except she understands that the noise is new and could be indicative of something failing.

Thanks! I love these automotive forums for getting help with problems like this, especially since I've only got a basic grasp of DIY car repairs.
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She's got an automatic trans, and it definitely happens when the car is in park right at startup, without her foot on the throttle. I have been in there when it has also happened while backing up slowly (like out of a parking spot), but again, right after startup with a cold engine. I don't think she's ever tried giving the engine a little rev to see what happens.

The few times I've been in the car recently, what happens is you start the cold car and the whole car has a slight vibration (like a slightly rough idle), and there's some vibrating/humming noise from what sounds like interior trim in the rear. The noise lasts 2 or 3 seconds and goes away, but you can still feel the slightly rough idle. When you start driving, either the engine smooths out a little, or we just don't notice it anymore. It doesn't make the noise at stoplights, parking lots, etc, once the car has been running a little. The noise occurs both when the car is stationary and when it is moving, but only right at startup with a cold engine (so she usually has it stationary still).

I drove the car some this weekend and tried to feel the rough idle at stoplights and when parking, but it is not so rough that it stands out. Just like you don't have any doubt that the engine is running, kind of. But no noise, and I still think the roughest vibration is right after starting the cold engine.

Thanks guys. Hope I didn't forget anything else important.
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Probably never. I'm not sure.
I don't know. Never seen the service schedule for her car. My bonneville manual said that the automatic transmission fluid should not need to be serviced or changed as long as it didn't look contaminated or smell burnt. So I didn't know she'd need that.

But that wouldn't explain a vibration or rough idle when she's in park would it? And the car doesn't exhibit any other abnormalities that I would normally attribute to the transmission. It shifts very smoothly and quietly, accelerates smoothly, etc...
Thanks for the information. My fiancee generally takes her car in for service (oil changes or specific problems) or has her dad do oil changes and simple work when she's home visiting. So she doesn't have a repair manual for her car. But we're 5 hours from her parents' house now, and we don't have money right now to throw away on shop repairs we could do just as easily ourselves. I like to do whatever work I can on my own car, but that generally has just meant brakes, spark plugs, oil changes, air filters, and other simple stuff. So before we take it in to a shop or dealership and end up paying for work we probably don't really need, we were hoping to get some information on here.

You have been very helpful with your suggestions. It's also great to see that one of the mods lives here in town. We're new to Tennessee, so that's especially helpful.

I think I'll go ahead and do the spark plugs and wires this week and see if that helps. As for the engine roll restrictor, I've read about that on here in my searches, but I couldn't think of a time she'd been through a lot of gravel and debris, and it sounded like that usually happens immediately before the problem becomes apparent. Still, she comes from a small town that probably has a lot of gravel roads and parking lots, so it sounds reasonable. I'll do a search for what that part looks like and where exactly it's located. If anyone can point me toward some pictures so I know what/where I'm looking for, that would be great.

Thanks again. I'm sure I'll have more questions for you in the future.
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Info no longer applicable. [unsure]
Ok so keeping in mind that this is not a 2004 like she thought, but a 2005... is this stuff with the roll restrictor still applicable? I'm gonna do plugs today or tomorrow, so hopefully the idle will be smoothed out, but I just want to check. Thanks! And sorry for the incorrect info.
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