67 390 4speed Transmission Knock?

I have an original 1967 390 V8 mustang with a 4 speed trans. While in the garage, engine running, I can increase the throttle then release it, I am hearing a single short knock sound on the driver side of the bell housing when the engine speed then decreases but does not occur at idle speed, somewhat above idle speed. Any ideas???

horse sence said:

About the only thing that could make a noise would be the throw out bearing ,unless the clutch fork came off the pedestal or the pedestal retainer broke, Does the knock go away when the clutch is pushed and you bring up the RPMs?

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So I fired it up today. Reved the engine and noise as usual was there on the engine decel. I pushed in and held in the clutch, reved, and no noise. Interesting part now, I slightly had my foot on the clutch pedal, pedal fully released, reved the engine, and the sound returned but I could feel a light jolt on the pedal as I heard the noise. Pushed and held the clutch, reved the engine, no sound. Repeated several times with clutch pedal held in, no noise. Again tested with no depressed clutch, and now the sound has gone away. Pulled the car in the garage, clutch works fine. What do you think, throwout bearing going bad? I have had several manual transmissions, never had a bearing go bad, only clutch plates that have worn out. But always have replaced all partsof course. Thanks.

horse sence said:

One more possibility ,the pilot bearing .Is it a bronze bushing type or a roller bearing ? If it is a bronze bushing it could have egged out .

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I was planning to remove trans for inspection either way.