Grinding noise only when accelerating

[Olds.PhD]

Need some help on a new transmission noise. I've had my Tremec 6-speed for a few years at this point with no issues. I recently swapped from my Olds big block to an LS. New bellhousing, new flywheel, new pilot bushing, reused the clutch (as it only had about 5000 miles on it), and reused the throwout bearing. I did check runout and it was way off - required .014" offset dowels. Got those in, rechecked and its good now (about .003 max variance on the dial). Rechecked my throwout bearing clearance, and its within the .125 to .250 gap that Silver Sport Transmissions recommended. Fluid was changed when I did the swap since I might as well.



Now, it has a horrible grinding sound only when accelerating. When backing out of the driveway, no sound. When coasting down the hill to get out of my neighborhood (up to 20-30 mph), no sound. As soon as I start applying any load, it starts grinding. At first I thought the driveshaft was grinding against something. But after the first drive I confirmed, no marks showing anything rubbing there.

Attached is a video. Also, you'll hear a ticking from the engine - haven't tracked down whether that's piston slap or a lifter. It doesn't really go away, so I'm leaning toward lifter. Pulled it out of a junkyard, so hoping driving it will pump it back up, but if not, new cam and lifters will go in. Not really as concerning as the grinding noise right now.



I'd rather not just throw parts at this. No problem replacing the clutch if that's what's wrong, but it didn't have many miles and was in good shape, so I eliminated that as an issue, but maybe I'm wrong?

[Timbers68]

I couldn't hear any grinding noise when you shifted. If it's grinding, you would be able to feel it in the stick as you shift. Simple test for the clutch, car running, trans in neutral, push the clutch pedal in, then shift into reverse. Do this in a three count, push pedal in - one, pull shifter towards reverse - two, put trans into reverse - three, if it grinds going into reverse your clutch isn't releasing. Maybe you didn't get the throwout bled all the way.
I hear a lot of other noises happening in your video, a lot when you shifted into 4th. Was none of these noises happening with the other motor? You didn't change the fluid type in the trans did you?

[Olds.PhD]

To be clear, the issue is not shifting. It shifts perfectly. The grinding noise is only when accelerating while already in gear - any gear. The "other noises" you're talking about may be what I'm talking about. The tick is obviously motor, the whine is obviously power steering, but the grinding noise is new, and sounds like its coming from the transmission area. The loud noise in 4th gear could be it, exasperated by lugging the engine a bit on that shift. It had a similar box of rocks rattle when lugging the previous engine, but no grinding noise going with it.

The throwout was never disconnected, so no change there. Fluid is Dex III, as required by Tremec.

[Timbers68]

Ok, I see what you're saying on the noise now. The biggest noise I'm hearing I would describe as a rattling noise, probably the same as your box of rocks rattle. This is why it's fun to try to diagnose noises online let alone in person.
I'm going do some spit balling here.
Almost sounds like exhaust hitting the floor or a really bad bearing noise. Tremecs have tapered bearings, they don't just go bad usually. Did you happen to drain the fluid out of the trans, did it look ok?
If you have a tin flywheel shield, I would look at that too. I've seen those make some crazy noises.
Are you motor and trans mounts in good condition? A bad mount can allow things to move and make contact with other things as you're accelerating.

I would put it up on a rack or blocks and run it in the air and have someone underneath to better locate where the sound is coming from.

Interested to hear what you find.

[Olds.PhD]

Yeah, its hard to get the sound and feeling to come through a video, especially with all of the other noise.

Fluid drained out looked clean.
Not sure how the scatter shield would cause this noise, but at this point, anything is worth looking at.
Motor and trans mounts are both brand new.

I have run it up on stands, but unfortunately, there's no load, so the sound doesn't show up. Makes diagnosis hard when the sound only appears when accelerating under load.

[Olds.PhD]

If you have a tin flywheel shield, I would look at that too. I've seen those make some crazy noises.

So, I started poking around a bit with the car still on the ground and ended up feeling like the clearance was somewhat tight between the flywheel and block plate. I took a look through a hole near the starter with my borescope. The clearance there seemed OK (best I can tell), but going through and looking back at the block plate from the rear I *think* I can make out what looks like gouging and I can definitely see that my yellow chromate finish is now bright blue, indicating its getting hot. So I think that's the sound - flywheel rubbing on the block plate. I'm guessing the way the engine is loaded on the thrust bearing changes when the car is coasting vs when the car is being propelled forward by the engine under load and that is why the sound is not consistent. Good call on that one!

Unfortunately, I'm having some work done on the garage, so I won't be taking this apart for the next few weeks at a minimum. No idea why things are contacting, but at least I have a real problem to work with instead of the nebulous noise.

[Timbers68]

I've been building transmissions for over 30 yrs., I've seen a lot of weird things. I've seen little tin flywheel/torque converter shields/block plates make some unusual noises. Tried to list everything I could think of. Sometimes it's the simplest thing.
I'm really glad you figured it out, and it's cool that you posted what you discovered. Win.