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Grisezd

Slow, rhythmic thrumming sound 50-60 mph

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Hi all. Had my new 78 brat out for its first highway-speed run yesterday. On relatively good asphalt in 2wd, about 50 mph I start to hear a "rum rum rum..." sound. If I put in the clutch it stops immediately. If I continue to accelerate it keeps going until about 65mph. Tires are new and balanced. Any guesses? I know it's some harmonic vibration but in what?

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Sounds like a through out bearing to me not a big deal unill it starts squealing they usaly hold till you replace the clutch usaly

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try puting a little presure on see if the noise goes away my outback did that i ended up using a motorcycle hand grip wedged between the lever and bellhousing to keep just a little bit of presure on the fork and bearing and the noise went away if its hitting then skiping then catching again it would cause a vibration of somesort but mostly just makes noise a slightly bent clutch fork could cause that most often its just a dirty bearing

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Just put a little testing into it.  It's not the clutch, no amount of goofing around with it changes the sound. 

 

Lug nuts are tight, wheel bearings feel good, nothing sounds right when I hit it with a mallet.  I notice that I'm feeling it too, through the body but not stronger in the pedals, shifter, or wheel.   I did notice when under the car that the left rear axle shaft slides very freely between the joints as I'd expect, but the right rear slides "gritty" and I can get it to stick in some angles of rotation.  I assume those should be very free to float. 

 

Does a failing axle make that sort of sound?

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The fact that the sound goes away when you press the clutch pedle would sugest that its not a axle or drive shaft that sound is most likely a bearing of some kind or gear noise unless it makes that noise in 4wd theres no load on any of the rear componets till its in 4wd so no reason for the sound to change when the clutch is pushed maybe its stuck in 4wd try changing the oil in the trans and rear diff and clean that axle so its not.gritty ive had vibration from failing axles but never any noise but i see how it could happen pushing the clutch wouldent afect that though what ever it is it dosent sound like a big deal i would drive it till what ever it is fails probly never will though fresh oil and greese will help even if it dosent make the noise go away its very posable you have a few problems the noise and vibration could be caused by different things

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I pulled the driveshaft anyway, felt clicks in it when I shook it by hand. A friend reminded me that I've got a few tweaked wheels, will try moving them around after the new u joints are in.

 

I think I overstated the clutch thing. It vibrates when I coast down through 55 mph with the clutch in.

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No, still happens. It’s dead cold out now so I’m just driving it and learning to love it until spring. Some folks on a Facebook group are certain that it’s a bent wheel and I do have at least one that’s not entirely square. In thee spring I plan to do front sus bushings and dig up some straighter wheels. Good luck!

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Ok, now I actually have an update. As the roads are cold and wet but not frozen I decided to try running with 4wd engaged on my commute. No rum-rum-rum noise at all! I confirmed that 4wd was actually engaged when I tried to coast into my parking spot and got the bind I expected.

 

A mechanic I work with suspects that there is a bearing that turns when in 2wd (separating the drive from the rear output) that does not need to turn on its own when 4wd is engaged. For those who have had the transmission apart does that make sense?

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I've never had one apart, so I'm not much help in your situation.

 

We had a nice few days here so I took mine for a drive and did the same test. Still vibrates. Mine is completely engine speed related. Sitting still in neutral it really picks up above 3k rpm whether the clutch is engaged or not.  I will add that right after I got the brat, it had trouble starting due to a worn ring gear, so I removed it and rotated it to where the un-worn teeth would do the engaging. It also had blown headgaskets and a worn thrust bearing, so it didn't get driven much before I had the bottom end rebuilt.

 

I have eliminated the water pump and alternator as a possibility by running it without the belt. The distributor shaft feels nice and tight still.  The tranny mounts are new, and the engine mounts are presumably original but not oil soaked or broken.

 

I have a vibration analyzer so the next step for me is to spend some time playing with it.

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Update. Ended up being the flywheel and/or pressure plate. I found a better flywheel somewhat locally and had it resurfaced and balanced with the pressure plate.

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And now an update on my original issue. Sort of... I pulled the front axle shafts and inner joints and all is well in the world. Noise is gone, shifters don’t vibrate any more. One or both axles were bad or badly unbalanced. Now running RWD and loving it!

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