knock, knock?

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Hi,

First post. Hopefully in the appropriate forum. I went ahead and read the 'read before posting' sticky and I'm sorry to say I don't know half the information asked (size of engine and transmission?). But if you notice my user name you'll understand why and be nice :)

Ok, so I have a chevrolet Z24 2000. It's an automatic. I bought it second hand 6 months ago from a 60-year old + woman who only used it in the summer. She had every single bill since purchase and 90% were every-3-month oil changes. Car had 45 000 km on it. Had it checked. Everything is perfect. It's been acting like a 45 000 km car driven by a 60-y/o woman who changed the oil like clockwork and never used it in the winter, until this week.

I'm posting here instead of going straight to a mechanic because last time I did that I almost paid 300 bucks for something that really only cost a pint of oil (thanks for telling me to check, dad!). If I were a mechanic and a girl walked in saying she had trouble turning the wheel I'd probably check the oil that makes the wheel turn before looking for parts that need changing. But then, I'd be an honest mechanic. I know all mechanics aren't dishonest and probably most aren't but it's unfortunately not written in front of their shop and I'm easy prey (30 y/o woman who has that deer-caught-in-headlights look at any mention of car function or part, aside from the basics like oil and windsheild washer). Maybe with your help I'll actually look like I know what I'm talking about when I go to a mechanic, as I'll inevitably have to soon, and won't be taken for a 'ride'.

So the problem: The car makes a knocking sound. I drove it a while today to pin down exactly when it makes it.

- One time out of 2 it will make one knock when I accelerate from a stopped position (eg. when the red light turns green);
- it always knocks (repeatedly and fast) when I take my foot off the gas, but;
- it only seems to knock that way when the car's speed has gone down to around 50km/h (I haven't been able to check it at high speeds like on the highway yet, but did manage to get it up to 70km/h in town, took the foot of the gas and knocking only began at 45-50);
- it will knock as it decelerates until it reaches 0km;
- the knock's 'speed' increases the slower the car goes;
- it doesn't knock when I brake;
- it doesn't knock when I'm accelerating from 0 to 50, or more.

The sound appears to be coming from the driver's side but it's really hard to tell. It could also be in the back. I don't know.

So there you have it.

I'd really appreciate any insight/advice you could give. I'll probably be bringing the car to a mechanic on Thursday, and no, I won't bring it to the guy who didn't check the oil first for the steering problem.

Thank you!

CM

posted by  clueless_mech

That's a good one. At first with experience #1 (Taking off from a complete stop) it sounded like it would be a control arm bushing. However it wouldn't knock at speed. Are you positive it's a knock? Maybe grinding, howling? I honestly am stumped if it's a knock. The only thing I can think of is broken brake rotor (RARE) or loose brake caliper (Not quite as rare). I can't say fopr sure what it could be, since I haven't seen/heard the problem personally.

Oh, and if I'm not mistaken, the 2000 Cavalier Z24 has a 2.4L Twin Cam inline 4 cylinder engine and a 4 speed overdrive automatic (Or 5-speed manual) transmission.

posted by  Satty101

My first thought was motor mount..



But that doesn't seem to go along with the rest of your descriptions. I am also a bit stumped. Could you describe the sound better? maybe an analogy: hammer hitting an anvil, bass drum, beating a pot with a spoon, etc....

posted by  corbett_auto

Hi Satty,

No, not grinding or howling. It's really a knock. What I find weird is the slower the car goes, the more knocks! So it's not wheel related? Say I was going at 60, take the foot off the gas, and the knocks start at around 45-50... they're maybe at 1 second intervals. By the time the car's going at 30-20 its just a series of knocks without much space between. I'm going to be driving it to work again tomorrow and try to figure out which side it's coming from. I also did a search for 'knocks' on the web and lots of car knock problems came up but none like mine. Many describe a knock when turning the wheel though. I know what that knock sounds like because I had it with another car (I forget what the problem or resolution was). It's exactly the same sound, though as I said, unclear about the location. At it doesn't happen when I turn the wheel, as it did for those people describing their problem on the net, or for my other car.

CM

posted by  clueless_mech

Hey Corbett...

Very hard to come up with an analogy. None of the ones you gave sound like it. Closest I can think of is maybe a wooden spoon hitting a pot... The sound isn't really metalic... more dull. But like I said in the other post, if you do a search for car and knock there are a lot where they say the sound only happens when they turn, and I remember having that problem with another car. It's exactly the same kind of knock.

CM

posted by  clueless_mech

The knock when turing is the CV joints in the front axles. And yes that usually only happens when turning. ANd the sound you hear is very similar to that?

So you say the knocking gets faster as you slow down? Just to test a theory try lightly applying the brakes to see if you can stop the sound with the brake pedal. Also does the car have a tach? see if the speed of the knock has any relation to engine speed.

posted by  corbett_auto

Maybe this is not the same problem, but my son's 99 Z24 had a sound best described as "a log rolling around in the trunk". It turned out to be a broken strut mount - a bearing covered in rubber that the top of the strut bolts to. His usually made noise only going over rough roads, but I would guess that anything that stresses the suspension ( like accel decel ) might also cause the strut to wiggle and make the thumping noise.

posted by  xsicdn

Very similar.



The knocking gets faster as I slow down. When I brake, it doesn't knock. I'll try breaking more lightly tomorrow. I don't remember if knocking resumes after taking foot off the brake. Will check on that too.



What's a 'tach' and by 'engine speed' do you mean the speed of the car, or something I should check on the RPM dial?

By the way, Satty:

I think you're right about the 2.4 because I think that's what the 24 stands for. As for the speeds, I can only downshift manually from 'drive' (it's an automatic) to 3rd, 2nd and 1st. I also tried downshifting today instead of braking. No knock when I brake; knock when I downshift.

xsicdn: definitely not a 'log rolling in the trunk' sound :-) Hopefully won't hear that sound in my future.

Thanks a lot for all your help guys,

CM

posted by  clueless_mech

Man...I'm interested to figure out what this is...it's actually starting to bug me a bit. haha

Clueless: the 24 in Z24 doesn't stand for the 2.4L engine. It definetely makes sense, but then the Beretta Z26 (3.1L V6), Camaro Z28 (5.7L V8...for the most part). Also, in the gears, the "D" is overdrive. Usually at higher speeds, will shift into overdrive (4th gear) to conserve gas and such. It's confusing, and I for some reason really can't find the words to explain. haha. :thumbs:

posted by  Satty101

Yes the RPM dial is the tach and it tells you how fast the engine is turning, which is different than the speed of the car.

posted by  corbett_auto

Mystery sound solved:

The part that supports the engine (engine support mount?) was broken (or used out). Ordered part: 'dogbone' support mount. Will be fixed Monday and I'll be out 190$ (CAD).

Thanks for your help,

CM

posted by  clueless_mech

Glad to hear it!:thumbs:

posted by  corbett_auto

After all that and it was a dogbone motor mount. haha

posted by  Satty101

I have a jeep and its making the EXACT same noise... never happens when I'm turning and usually between 40-60 km when I take my foot off the gas.. Literally a knocking sound.... I had my ball joints replaced and it was great for about 2 weeks Im wondering if that might have something to do with it

Chad   10 Dec 2012 07:36

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