Manual Transmission - Please help me diagnose T56 rattling

I have a T56 which I had rebuilt in conjunction with an engine upgrade. There were no major issues before the rebuild.

I installed the transmission and drove for about 30 miles but then discovered a leak at the tailhousing. I removed the tailhousing and fixed the leak. I'm 99% sure I reassembled the tail housing correctly--from what I gather as long as you get the 5/6 rail in place then it's good to go.

After reinstall the transmission exhibits a rattle at idle while OUT of gear with the clutch engaged. I can make the sound go away by disengaging (pushing in) the clutch. The rattle is synchronized with the lope of the engine. It cycles louder and quieter about 1-2 times a second. The rattle's frequency is about 10-20 cycles a second, like a vibration but with a hollow metallic sound.

The noise does NOT go away when I hold the shift lever, or even if I press the lever left and right. The noise does not seem to be coming from the shifter mechanism at all, but it does sound like it's coming from the rear of the transmission.

It seems like the sound goes away once the car is moving, although it's hard for me to hear the rattle over the engine/exhaust sounds.

I'd appreciate any ideas as to what the problem could be, or methods by which I can narrow down the cause. Thanks in advance for your help,

-ch

Do you have an agressive clutch? What about an agressive cam?

Pilot bushing/ bearing
does it sound like clutch chatter?

Its the normal sound that most have with the T56. It sounds like pebbles in a shoebox. It happens because of normally worn clearances that open up over time with bearings and gear lash. Engineers call this gear clash. It only happens when the tranny gears are not loaded.

If you have a non stock lumpy cam, that will intensify the problem. Mine does it at 800 RPM. If I raise the RPM to 900, it is less pronounced. The Corvettes had the dual mass flywheel which also helps to reduce the sound.

The noise is only an annoyance and is totally harmless.

Thanks for the responses. I've replied below:

Do you have an agressive clutch? What about an agressive cam?

The clutch is the same one I had before the rebuild. It's a Spec 3 and I've had no issues with it so far. The cam is a Katech torquer which is not particularly aggressive and is used in many of their street cars.

Pilot bushing/ bearing
does it sound like clutch chatter?

The pilot bearing is new, and the sound only happens when the input shaft is out of gear. I'm fairly certain it's not the clutch I'm hearing.

Its the normal sound that most have with the T56. It sounds like pebbles in a shoebox. It happens because of normally worn clearances that open up over time with bearings and gear lash. Engineers call this gear clash. It only happens when the tranny gears are not loaded.

If you have a non stock lumpy cam, that will intensify the problem. Mine does it at 800 RPM. If I raise the RPM to 900, it is less pronounced. The Corvettes had the dual mass flywheel which also helps to reduce the sound.

The noise is only an annoyance and is totally harmless.

I will do some experiments varying the idle and see if this has any effect. It's nice to learn that this may not be a serious problem, but I'm stumped as to why this would happen after a rebuild. The cam I'm using in my new 427 is more mild than the F13-114 I had in my 346...

-ch

So I did a little testing today. I let the car idle on jackstands. Here's what I can say:

1) The noise sounds like it's coming from inside the tail section. I can hear it from under the car. It's loud enough to be heard over the longtube headers.

2) If I rev the car above idle (950 RPM) the sound persists, although it cycles faster.

3) If I remove the shift knob, the sounds gets quieter. However, if I hold the shift lever (or press it side to side) the sound does not change.

The transmission had only about 40k miles before the rebuild (which was done in preparation for my 427). I want to be absolutely sure that there isn't something broken inside...


-ch