NEED HELP

I don't agree with the TB theory.

I suspect what you're hearing is normal gear noise. When the engine is turning and the trans is in neutral there's still a bunch of cogs and junk turning in the gear case. Minute changes in engine speed at idle can cause the gears to knock back and forth a bit in their backlash and you hear this as a sound from the gear box. It can be exacerbated if the car has a lightweight flywheel or cams and the idle is rougher than OE. (You should hear a VW O2J 5-speed transaxle with a light flywheel...)

A bad TB is characterized by lots of noise -- squeal, groan, screech etc -- and perhaps "grumbling" felt when you press on the clutch pedal. But the bearing is doing pretty much nothing and under zero load when the pedal is released.

trinity_gt said:

I don't agree with the TB theory.

I suspect what you're hearing is normal gear noise. When the engine is turning and the trans is in neutral there's still a bunch of cogs and junk turning in the gear case. Minute changes in engine speed at idle can cause the gears to knock back and forth a bit in their backlash and you hear this as a sound from the gear box. It can be exacerbated if the car has a lightweight flywheel or cams and the idle is rougher than OE. (You should hear a VW O2J 5-speed transaxle with a light flywheel...)

A bad TB is characterized by lots of noise -- squeal, groan, screech etc -- and perhaps "grumbling" felt when you press on the clutch pedal. But the bearing is doing pretty much nothing and under zero load when the pedal is released.

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I agree.. Sometimes even an A/C compressor can introduce a vibration thru the crank into the trans and cause it to rattle/knock. Make sure it's full of good lube and keep an eye on it. Don't tear in to it yet.

I've never heard someone describe a bad tb as a knocking sound.

If that's really what it is, just deal with it. I've driven tons of miles on many different cars that had tb making noise.

Actually, I just re-read what you said in the OP after reading your reply and notice that you seem to be saying that this noise goes away even with just a little pressure on the pedal; that is, it doesn't have to be fully depressed. If this is the case then it's not "normal" gearbox noise you're hearing since the gear box is still connected to the crankshaft via the clutch.

You might have runout in the diaphram spring of the clutch that's causing the TB and release arm to move around a bit. Putting some pressure on the pedal puts a bit of preload on the spring, release bearing and the arm and quiets things down. Get under the car (jackstands, watch hot exhaust parts etc) while it's idling and eyeball the release arm. How much is it moving? Can you put your hand on it and feel the "knocking"?