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I picked up a 97 civic ex (manual transmission) for next to nothing. It has low miles and is flawless other than a banged up rear bumper and this horrible, loud , grinding sound coming from the underneath, front center of the car . The noise only presents itself when under acceleration. In neutral is silent, with the clutch pressed in its silent . So I can be doing 50mph with it grinding like crazy, press in the clutch and it goes silent. I've driven the car roughly 30 miles this way in order to get it home and it drives great. It's just loud and sounds like something is being violently destroyed under the car. I bought this for my son hoping I could get it fixed relatively quickly. I haven't even gotten under there yet, I was hoping to gets some ideas here first . Thank you all for your time.

 
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Get under it and look first. Nobody here can see what's making noise under your car.

We can guess all day long.
I'll guess Professor Plum, in the study, with a pipe.

 
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Allow me to rephrase.... Is there anything that could happen by a rear end collision to cause this noise ? It's not in the wheels.... It seems to be something internal with the drivetrain or differential. Looking under the car probably isn't going to yield a lot of results. Also, I'm not asking you to guess at anything. If you have dealt with this or have knowledge of how the drivetrain works then perhaps you can share your thoughts. I know if you were to tell me your right front wheel is making a "whirring", grinding noise I wouldn't need you to remove the wheel and look at it before i could advise you that it may likely be a worn wheel bearing . I'm simply hoping maybe someone out there recognizes the description of my problem and can chime in with their experience . I'm not asking you to fix my car. I'm not forcing you to contribute if you don't want to.
I frequent and contribute often on several BMW forums and I'm just always amazed at the people who take their time to write some completely pointless response like "take it to the shop" or "I have no idea" or "I'm guessing Proffeser Plum"..... Look, I'm new to this forum. Maybe you all are a tight knit community as we are at the BMW forums and I'm about to be run outta town for this comment. I just had high hopes for this site after reading so many helpful responses in some of the threads I scanned through. Then to have to first response to my first thread here be "I'm guessing Proffesor Plum" is just a huge disappointment . Sorry to waste your time....

 
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You left yourself wide open.

Due diligence, a cursory inspection, looking for a likely cause prior to posting is kind of expected. You stated you hadn't even looked under the car yet in the first post.
An awful lot of people will make pointless posts asking questions such as "My car is leaking something, what could it be?" (Answer: It's broken.) Then we find out a bottle of pancake syrup spilled in the trunk, or something equally ....inane.


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How bad was the collision? How much damage, and where? Is the damage only cosmetic, or is it so bad the trunk now sitting where the back seat used to be?

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Your description of how and when the noise occurs made me think of a transmission bearing going bad, possibly in the differential, but that would not have been the result of any rear-end accident.



Exhaust bent and making contact with the body or suspension or something along those lines, I could picture as the result of a rear collision. Exhaust will move as the engine moves, so it seems quite possible.

Likewise, anything attached to the engine or transmission that makes contact with the body during acceleration could be suspect......but we can't see or hear your car.



Without more data, all we can do is guess.
You, or a trusted mechanic needs to put eyes and ears to work.


HTH

 
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Ezone - Fair enough. I appreciate your help. My earlier comment was a mixture of a rough morning and my faulty nature of assuming the worse.
The trunk is "slightly crinkled" and the rear bumper is pretty smashed. The trunk closes with careful force. The rear doors are tough to shut all the way. The car seemed to be hit pretty hard, but the airbags didn't deploy so it's hard to say. The theory of a transmission bearing was my first guess, but (and correct me if I'm wrong) the throw out bearing is noisy when the clutch is depressed and the output bearing is noisy when you let off the clutch. This noise only presents itself when under acceleration. So if I'm doing 50mph with my foot ON the gas it's making the noise. If I take my foot OFF the gas it's totally silent. The clutch being in or out is irrelevant . I plan on getting it up on ramps for a visual inspection this weekend, but I really need some dependable jack stands so I can put it in gear and accelerate. The reason I'm thinking differential is because (and I'm very inexperienced with tranny and drivetrain) I thought the inner gears (pinion gears ???) engaged when under acceleration only. I realize we are simply talking theory here and I am just hoping to be armed with knowledge to know what I'm looking at when I look. Thanks again !

 
Rep Power: 517

Has this car been seen by a good bodyshop?
Has this car been totaled?
If the rear was hit hard enough to buckle the body and make doors hard to close, that's probably a pretty bad hit.

Yes, and there is also a pilot bearing that could make noise under a nearly identical condition.

No, not an output bearing. There are many different bearings inside the trans. Careful identification of the conditions under which the noise occurs, and the pitch of the noise, and how the pitch changes with various driver inputs leads to identification of a likely suspect bearing or gear area. Once this is determined, one should then know what areas to look at inside the transmission after it has been removed and disassembled.

Pitch: Does the noise change with road speed, or does it change with engine speed? Does the pitch change stay present throughout a single gear, or does it repeat with all gears? Some gears? Is it the same pitch in each gear at a given road speed?
55 MPH in 4th and 3rd gears produces the same pitch of the noise as 55 in 5th gear? Different?
(It's hard to describe all the different ways within a single question)

Input shaft bearings are the usual culprits heard with the clutch pedal out (foot off, clutch engaged) and trans in neutral.

IF the noise is transmission related, that description pretty much rules out clutch, throwout, and input shaft items entirely. Countershaft, differential, or maybe you got a huge rock caught between the trans and the subframe, or .......

Is the trans full of fluid? Is it leaking from the axle seals?


Scratch all this, my fingers hurt.
You say there's a noise, and describing a noise...ain't the same as hearing it.
Got a video of the noise, along with showing how you are driving when the noise is present?
Keep in mind that phones and laptops usually suck at reproducing sound. I know my laptop sucks.

 
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Going to make a video today. Thanks for your help ����