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Purchased my 98 Civic DX in May with 94k on it, 112k on it now (lots of highway driving). Had to replace an alternator and an o2 sensor soon after I purchased, but everything else has been peachy.

This is my first winter with the civic. Temperatures are fairly mild here (low 50s/high 40s). Lately, when letting out the clutch while changing gears, I'm hearing a strange rattling sound that seems to come from behind the steering wheel/dash. Reducing throttle stops the rattling sound or quiets it considerably. The rattle isn't terribly loud, but noticeable, and does not seem to be rhythmic/directly related to engine speed. Revving the engine in neutral produces no noise. It's audible right after letting out the clutch and accelerating on a gear change, can be worsened by pressing hard on the gas, and is also more noticeable when driving up hills in lower gear. If the clutch has been engaged and I'm just cruising, the noise seems to be gone, even if I suddenly accelerate. The noise is most noticeable during the first 5 minutes of my trip, but seems to be getting worse. I've tried to find audio examples of what detonation would sound like inside the cabin, and while I can't completely eliminate this as the cause of the noise, I'm pretty sure it's not what I'm hearing. Putting higher octane gas in the tank didn't seem to make a difference.

I'm thinking I may need a new clutch anyway, as I'm not sure when/if the clutch was last replaced, and the pedal seems to travel much further before the clutch engages now. Smooth starts have lately been troublesome as well. Any thoughts?

 
Rep Power: 235

sounds like your throwout bearing is bad. if you sit in neutral foot off clutch you hear it rite. try pressing the clutch in see if the noise changes.

 
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^^i would agree. had the same issue a couple years ago. park next to a wall and really listen for it.

 
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Actually, no, the car sounds normal in neutral with my foot off the clutch. The noise is only noticeable during the acceleration/gear change process, especially between 1st, 2nd, and 3rd gears, and seems to worsen with more aggressive, quick shifting. It only happens while I'm letting out the clutch and giving the car gas, and immediately after, especially if I'm accelerating quickly or going up a hill.

Its intensity also seems to correspond to engine pitch. (The DX doesn't have a tach or I'd note RPMs.) The rattling starts when the engine is at its lowest note right after the gear change, and continues to intensify during acceleration until the engine reaches its typical note/optimal speed for the gear (for example, around 35mph in 3rd), at which point it seems to go away.

If I'm driving in a particular gear for awhile and I don't have to shift, hard accelerations don't cause the noise anymore. My car seems a little sluggish on acceleration too, but then again, the DX engine is lame on acceleration anyway.

The noise kind of sounds like loud chattering teeth. There's no squealing or grinding. Do you still think it's the throwout bearing, or could it be something else? Thanks for your help!

 
Rep Power: 235

well my throwout bearing was screaming whenever i had my foot off the cluth, so yea its possible its that. but there are also many other things it could be. you could also have bad bearings in the trans, try draining your trans fluid into a clear plastic bottle and then look at the color of the fluid with a flashlight see if there are any metal particles in the fluid, if the fluid looks like it had lots of gold flakes then you are chewing up your shift syncros. its hard to diag a noise through the internet lol. but let us know what color/contents of your fluid if it comes out normal color like it would look from the bottle then i would lean to the throwout bearing. but take it one step at a time.

 
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Turns out it's not the clutch - it IS detonation! Surprise!

At least it's not the transmission. I'm surprised that the check engine light didn't come on - isn't the knock sensor supposed to detect detonation/pinging too and adjust accordingly?

This started happening randomly. Timing belt was changed at 93k and detonation was never an issue. Just now started happening at 112k. Mechanic suggested it could be carbon deposits, and that I drive the engine at higher RPM for awhile with a tank of high octane gas to try and burn them out. He said unless it gets really pronounced that I shouldn't worry about it. All I know is that my car wasn't doing this before, and that the civic shouldn't ping with regular gas.

Could this be indicative of another problem, like something in the ignition system failing?