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I am trying to identify a noise that I couldn't find any existing useful posts about. My 2005 Civic EX 5spd has 50k miles on it and runs perfectly. There is one noise that I noticed lately that bothers me a bit.

When I accelerate in 1st, 2nd, 3rd, it makes a single knocking sound when the engine passes ~2k rpms. When I decelerate and it falls below 2k rpms, it makes the same sound. I have no issues with turning or braking, the clutch seems fine (no engagement issues or noises), the car accelerates fine, the tranny seems fine (doesn't fall out of gear or anything) so I can't figure out what it would be.

Is there a reverse or 1st gear lockout mechanism in the tranny that may be engaging? Maybe low tranny fluid? I saw someone mention a motor mount, but I can slam the throttle open at 3 or 4k rpm and not have any noises. AC works fine. The only other related/unrelated thing I can think of is that something (a belt?) whines a bit when the car is cold. If I accelerate it stays at the same pictch. When the car warms up, the noise is gone.

Thoughts?

 
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Actually, was driving around tonight trying to figure this out. 2k rpms isnt the sweet spot. If I jump on the gas in first at 1200rpms it makes the noise, if I get on it from no throttle at 2500rpms it also does it. It seems to make the noise as soon as I get on the throttle in low gears, and when I'm off the throttle as the engine settles back in below 3k rpms somewhere.

Maybe it is a motor mount issue?

 
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I have the same problem in my 04 EX coupe. It started happening around 40k, When I am in 1-3, at higher revs and let off the gas, the noise gets louder around 1800-2200 rpm. Once I push in the clutch, it goes away. I haven't had any mechanical problems with mine. I really think it is something to do with the exhaust system.

 
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Thanks for the reply - I checked the motor mounts and they look fine. Seems like the noise only happens when the engine rocks upon accelaration and upon the depressing of the gas. Other than the mild noise inconvenience, everything about the car works like new.

 
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mine does the same thing started at about 60K any clue to what your issue was?

 
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No clue - I have to believe someone on the forums has experienced this and successfully diagnosed and corrected the issue....

 
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Are you sure its not the engine knocking?

 
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i know its a bit late but I'd say its the EXhaust manifold gasket thats shot or the manifold itself is cracked. the noise would come from the vibrating gasket etc... happened to me recently. gasket changed and its back to perfect

 
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I had this rattling problem start at around 100,000 miles. I now have around 108,000. I have a Kamikaze header, no manifold. I don't hear the noise when revving the engine, only when the clutch is engaged. I was told it might be the throw-out bearing, but not sure. It drives fine, no real problems other than that annoying noise. If one of yall figured it out, please post the solution.