2004 Hyundai Sonata Vibration and Loud Engine noise when accelerating. Issue has been resolved!

I own a 2004 Hyundai Sonata (2004 Base model L4 - 2.4L 2351cc - MFI GAS DOHC) which is a 4 cylinder and has about 113000 miles. Anyways, about a month ago we had one of our coldest days of the year. I drove the car to work and everything was fine. Then when I left work (it was freezing cold) I went to start the car and it sounded like I started a race car. It was so loud that I thought the car was going to blow up (no check engine light came on). I changed the oil and that didn’t help and so I figured it was something with the exhaust. Then I took it to my mechanic (applause). He said that he was stumped, the car was noisy but he didn’t think it was a problem but he said he would try to figure it out. He ended up taking off the timing belt and said that was fine, he said the water pump was good as well. He had a service tech look at it with him and they thought the issue might be the “vibration dampener” or “damper” So they replaced that but it didn’t help. He said it could possibly be the motor mounts but he said when he looked at them they looked okay?

I drove it off his lot yesterday and it is quiet when I idle but I can see the steering wheel shaking significantly. Then when I hit the gas it is still obnoxiously loud and with heavy vibration! But when going down a hill and letting off the gas there is a brief respite of silence (still vibration). It is especially louder in lower rpms and sometimes I think I can hear a metallic clanking/scraping noise when I have the times of "silence" but it is very faint. 

Please mechanics, car enthusiasts, and intelligent car people please help!!! I might try to upload a video with the sound if someone needs to hear it.

Is this a manual shift or an automatic?

A mechanic does not remove a timing belt to inspect it. Perhaps he meant he inspected belt and verified proper cam timing? Cannot believe 2 mechanics could not pinpoint major engine noise. Might be a broken valve spring or cam follower. Something that would allow motor to run but barely and make a lot of racket. A bad bearing rod/crank would almost certainly make motor NOT run. Don’t think ur motor has timing chain? A bad tensioner is very noisy and common n lots of GM 4cyl motors.

In addition to the normal timing belt, this 4-cyl engine also has a separate belt-driven balance shaft. If that belt skipped a tooth, your engine would get lots of vibrations - (and yes it would keep running because your valve timing would be unchanged).

When you had your timing belt last replaced, did they also replace the balance shaft belt?

Start by changing mechanics.
If my understanding of what’s been checked and changed is correct, the next place I’d look is the engine/tranny mounts. A rocking engine could be opening up a tired header pipe joint.

It’d be easy to check. Put leather gloves on for safety, start the engine, and try with your hand to rock the engine. Use both hands even. Rock that sucker good. You won’t break anything. Just be careful not to get your hand in anything spinning. Also have a friend rev the engine while your head’s under the hood. You might get lucky.

Post the results.

I agree with mountainbike - check the more simple things first.


+1 @tsm,

Start by changing mechanics.

Obviously this one has used up his/her trouble shooting resourses.

Thanks for all your help. It is an automatic “researcher”. Also, I don’t believe the balance shaft has been replaced “Joe Mario”. I know another guy that works at a junkyard that is a whiz with cars. I’ll try to have him help me take a look at the engine/tranny mounts. Unfortunately it is the middle of tax season for me as an accountant and my wife is due with our third child on April 12th. My mechanic did assure me that I could drive the vehicle (And I can’t change mechanics because he is good and honest). Again, what Joe Mario mentioned about the belt for the balance shaft sounds very plausible. I will try to give my mechanic a call and double check with him on what he checked. Then I can give more accurate information. I will definitely give an update on what I find out. Thanks again guys!!! Also, It isn’t quiet anymore when I’m idling…

This is very murky but maybe an engine miss due to a failed spark plugs, coil, or engine mechanical issue?
A sound video might help.

Going into an engine for a timing belt inspection does not sound like a good way of going about things if this was done just to inspect belt condition and assuming that has anything to do with sudden poor running.

I have new timing belt and spark plugs so I don’t think that’s the issue. I think that the engine would act a little differently if it was misfiring and my check engine light would probably go on. I will try to upload a video file this weekend and I will try to call my mechanic tomorrow afternoon. I will update soon.

Oh and Joe Mario, I checked my service record for when they replaced the timing belt and in parenthesis the bill says (does not include balance shaft belt). I will ask my mechanic about that.

I’m surprised they didn’t replace the balance shaft belt when they did the timing belt.

Replacing the balance shaft belt is expensive. As others have noted, please make sure the more simple items have been checked first.

Hey everyone here is an update on this ongoing expensive car issue. I have to give my mechanic props and I’m sure I don’t explain things correctly on here so it makes him sound bad but he is seriously a good mechanic.

He told me that not long ago he has service records where he replaced the timing belt, the balance shaft belt, the power steering belt, the alternator belt, the spark plugs/boots. He clarified that he thought the timing belt was fine but he thought that the belt might have “skipped a tooth” or that it somehow got out of alignment (again I might be misquoting him). So apparently he already replaced the balance shaft belt. He said that him and the other mechanic discussed the balance shaft rod as being the potential issue but he said he wanted to check to see if it might be something else first. He said it would be unusual for the rod to break but that might be what the problem is. He will look the car over again on Thursday for me for no charge and then he will let me know what he thinks could be the issue.

He said it could be the catalytic converter possibly or maybe the motor mounts if not the balance shaft rod. I will also mention the things posted on here to him to maybe help him determine the root of the problem.

!!!Final Update and Resolution!!!

First of all my mechanic is the bomb! He gave me a ton of free labor and then he found out that the issue was the harmonic balancer (crankshaft damper, crank pulley damper) He replaced the whole crankshaft pulley and the issue has been resolved.

Thanks again everyone and hopefully this post can help some others with a similar issue.

Congratulations on a happy ending and sincere thanks for the follow up post.
We try to help people, but far more often than not we never hear the endings. It’s great to hear a happy ending.

Sincere best.