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2WD 2006 LLY Allison 6 speed...147K miles on truck.....75K on current fluid/external filter.....internal filter original

News you don't want to hear when you get home from work....my daughter got my attention about a noise she heard inside the truck while driving 40mph or higher. She described it as sounding like a locust in the truck and she wasn't kidding. No error codes, truck operates and drives fine. I thought maybe it was the beginning of the carrier bearing going out (changed it not too long ago as rubber was gone) but I had to dry through some fairly deep water in the street a few days before this sound started so that cued me in on water in the bearing.

After looking under the truck and rolling the driveshaft by hand there are no issues with the carrier bearing or any of the ujoints that I can tell without taking the shaft down to further verify. But I did hear something up in the converter location. With the truck in neutral (rear wheels off the ground) when I slowly spin the driveshaft I can hear a slight metallic scraping sound of something making contact with something else.

The sound is not continuous.....I hear several little scrapes, then it is quite, then several more; all in one rotation to the driveshaft (2 events of noise). I assume that the input shaft will spin the stator in the converter as I rotate the driveshaft. Happens in either direction. I pulled the little inspection plate off and put a stethoscope on multiple areas of the transmission and on the side of the converter. I can hear it the best on the converter which indicates to me the stator is moving. The little sounds could be the a few fins rubbing on something, then happening again a half rotation later? Bent stator?? Slack in the clutch plate for lockup (if there is one)?

Finally I decided to start the truck and bring the RPM up to listen. Could not hear the sound we heard while driving while in park (didn't check neutral). I ran the truck through the gears (manual mode) targeting 1500 RPM in each gear until the speedo reached the critical minimum speed of 40 or more. The sound did not begin until 4th gear. I thought I might have heard a slight noise in 3rd, but after 4th gear engaged the noise was very apparent. Could hear it in 5th as well. Didn't bother going in to 6th since we heard it on the road as well in 6th.

I did put the transmission in neutral and shut off the engine to allow the drive train to spin down so I could hear it better and the noise is too loud to point to a specific location (echo off ground and bottom of truck). Though I still need to verify the carrier bearing is okay by dropping the shaft and feeling it spin to completely rule it out.

The big concern I have is the sound I heard in the converter when turning the shaft by hand. Could it be the lock up clutch is worn out?

Fluid level is good. Fluid color...you can tell it isn't new but it is not burned. There is about 75K miles on the external filter and fluid. Internal is original. I plan to move forward and change the internal & external filter. Plus see if anything is in the bottom of the pan.

In the meantime, has anyone encountered something similar to this before? Advice / help appreciated. Cheers, Brian.
 
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