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M5OD crazy rattling noise

patternfall

New Member
Joined
June 20, 2010
Messages
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City, State
Seattle, WA
Year, Model & Trim Level
1990 XLT
Video w/ the noise here: http://www.youtube.com/watch?v=fLSaX7fmghA

At least, I THINK it's coming from the transmission. I've had a long and storied history with this car and transmissions... Got it off Craigslist, after ~1000 miles the transmission blew on the freeway. I had 4th gear but no others (it would shift into them but the car acted like it was in neutral). Removed that transmission and replaced with a donor. I apparently did not seal the top cover correctly (though I did plug the three shift rail holes w/ freeze plugs) and ended up with a persistent leak that caused another low-fluid situation and gave me a permanent whining noise. Got a deal at a dealer shop and had them seal it correctly (and also replaced the clutch).

That was probably 15000 miles ago. Now I have this knocking/rattling noise. It started very quiet. I could only hear it on acceleration and even then just a faint whisper. That was a week ago. My girlfriend has been driving it and when she brought it to me today it's what you see in the video above.

Two things:

1- The rattling gets MUCH WORSE under acceleration (though there are no changes in performance as of now)

2- The only time the rattling goes away is when I have the clutch in and it's in gear OR WHEN I'M DRIVING IN 4th GEAR (direct drive).

Is this another dead M5OD I have on my hands?
 



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So hard to tell from the video. To me it sounds like an exhaust leak, but I'm sure it sounds different in person:dunno:
 






That was what one guy said just by hearing it, but the real tell is how it completely stops when you just throw it in gear. Nothing in the engine or exhaust changes, but it COMPLETELY stops the noise and purrs quietly.
 






Maybe it's a broken clutch plate? When it's disengaged it's rattling around and when engaged and in gear it's fully compressed so the noise stops. Just a guess.
 






So if the clutch pedal is pressed but the shifter is in the neutral position and you're coasting it still rattles? Or was the condition at a stop?

And just so I understand correctly, if the clutch pedal is pressed with the shifter in any gear position while coasting it doesn't rattle? Or was the condition at a stop?

If I had to guess by sound alone it sounds like a bearing/slop inside the transmission. The fact that it goes away in fourth suggests it's internal and not a bad clutch plate or release bearing to me, probably where a gear is allowed to rotate on a shaft but rotates with the shaft in fourth? If pressing the clutch pedal at a stop doesn't immediately stop the noise but then placing the transmission into a gear position while still stopped does get rid of the noise I'd say it's internal.
 






If the car is in motion, it will rattle no matter what unless it's in 4th. Neutral or any other gear engaged, clutch in or out, doesn't make a difference.

I think I've gotten this down, and it's that I'm screwed on this. Taking it to a repair shop tomorrow to see what the $ damage is.

I hate these sensitive M5ODs so much. It's like they break if you yell at them harshly.
 






Hate to say it but I'm almost certain it's internal, probably where the fourth gear sits on the shaft, I don't have a breakdown to see how everything fits together though. Those M5OD's do seem to be pritty picky. I had an R2 in a F150 that managed to preform flawlessly yet a F250 I picked up would grind into second and pop out of fourth if you didn't hold the shifter. Engineered well enough to do the job and fragile enough to fail as soon as you pass their limit I guess
 






I hate these sensitive M5ODs so much. It's like they break if you yell at them harshly.

Mine has been thoroughly abused and has been great (knock wood). I run nothing but Mobil 1 synthetic in it. Bought the truck new in '93. The original tranny had problems within the first few thousand miles. It was replaced under warranty. No problems since.
 






I hate these sensitive M5ODs so much. It's like they break if you yell at them harshly.

...Or let them get low on fluid. ;)

My best guess is you have a bearing that's decided it's had enough (or maybe a gear that lost a tooth). Time for a teardown (or better yet, another donor if it was initially damaged 15,000 miles ago by insufficient lube).
 






probably where the fourth gear sits on the shaft

There is no fourth "gear" in these transmissions. 4th just locks the input and output shafts together.
 






There is no fourth "gear" in these transmissions. 4th just locks the input and output shafts together.

Aha, that makes sense! I just looked up a flow chart for the M5OD. If Im looking at this right in any other gear the input shaft would turn at a different rate of speed from everything after the first gear. Could the noise be the 3-4 synchronizer?

http://144.162.92.233/faculty/mwhitten/presentations/2313/36s03s0.pdf About halfway down was the manual trans stuff. Kinda handy
 






Hey diggin up an old thread i know! but i have the same issue with my transmission too! its annoying.. sounds like a low speed 1" impact gun... but im just curious if you might have gotten this fixed? what was the problem and how much did it set you back?.. in the mean time ill keep searchin
 






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