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Discussion Starter #1
Ok, I found a thread from 2006 with almost the same problem, but only 2 responses and not much information. Other hesitation/knocking issues were mainly engine problems that I found. Here's my issue.

I have a manual transmission 2003 ZX3. When I start my car, turn it off or let it idle, a fairly loud knocking noise is coming from the transmission area. I went under the car with it running to hear where the noise is coming from. During cruising speed with my foot on the accelerator, no noise, clutch catches fine and everything seems normal. There's no slippage.

If I disengage the clutch to change gears or let the car cruise and push on the accelerator, it takes a split second for it to catch. The car is extremely jerky when I have to maintain speed and keep my foot on and off the gas when following a car. At first I thought the noise was coming from my engine, but with it running, it really seems like it's coming from the tranny when I go under the car. Any guess as to what would cause this? Thanks for the help.
 

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Discussion Starter #2
Bump. Has anyone else had something similar happen?
 

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OK, do this test for me: idle the engine, clutch pedal up, trans in N. Listen for your sound, now push the clutch in. If the sound disappears, then that's normal. It's the throw-out bearing rattling between the pressure plate forks and the slave cylinder. There's nothing wrong with that. Mine has been doing it since about 10k miles and I have over 130k on the same clutch. It's what happens when a metal part is glued to a plastic part. Eventually, the metal part comes loose, and then it rattles a little. When you push the clutch in, the slave pushes the throw-out bearing against the pressure plate forks, and the sound goes away.

As far as the acceleration drag you're experiencing, I have no idea. I suggest cleaning the TB- winter is coming, cleaning the MAF, check and re-gap the plugs, inspect plug boots (replaced if needed). Do you still have the factory air cleaner? If so, perform the flow test as described in the owner's manual.

The jerkiness is due to a warmed friction plate in the clutch. Nothing wrong with that either, just slip the clutch slightly more in stop and go traffic.
 

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Discussion Starter #4
Ok, I tried those couple things. The noise didn't change at all with the clutch pushed in. It still made the knocking noise at idle. When I turn the car off, the noise is more pronounced and I hear it knock louder about 4-5 times before the engine cuts out completely. The same happens when I turn the car on. It knocks until the engine is on and the rpms go up. Then when it hits idle, it comes back again. It really sounds more like its coming from the tranny than the engine itself however. The only thing that worries me is that I have more going on than just the clutch. I've owned nothing but manual transmissions my entire driving career and never had a sound like this happen.

I'm more concerned about this noise than the jerkiness of the clutch. I can get rid of the jerkiness by giving it a few more rpm's than usual.

Plug wires only have about 10k miles on them and the plugs I just bought this year. They have about maybe 3K miles on them. I gapped them already as well. I change them out more frequently than normal because I buy the copper plugs. I have the JRSC on it. I also cleaned out my TB in February because it was sticking. I don't see any carbon built up inside it now.
 

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Discussion Starter #5
I thought I'd update this thread since I figured it out and other people might benefit while doing searches. It turns out that my rear motor mount bushings completely broke apart and the motor was only being held on by the upper two. I had an aftermarket rear mount on there (not even that old either). I didn't notice it before because the bushing hadn't completely broken apart yet. When I went under the car the other day, the mount was completely loose and bits of the bushings were crumbling by just touching it. I put the stock one back on and problem fixed. It must have been torquing the engine around until it wouldn't move any more, then catching creating that split second pause before the bang and wheels moving. That would explain why pushing in the clutch didn't change the sound coming from the tranny. That noise around the tranny was just the mount hitting it.
 
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