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Discussion Starter #1
When shifting, whether typical everyday shifting or spirited driving, I've noticed a CLUNKING sound from underneath. It usually only happens in the lower gears. It doesn't happen all the time either.

I did a quick search I didn't find much about it, but I did find reference that the rear differential could make this noise if there are teeth broken from it...?

I know nothing about this matter, so any advice on it would be most helpful.

I also have no idea if this is normal or not. This is my first AWD (MY99 RS)...to me it doesn't sound normal.
 

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Nox, sounds perfectly normal to me. The rear diff is just doing its job. You may want to consider STI rear diff mounts, tranny mounts, motor mounts to stiffen up the drivetrain but really, its quite normal to hear the *thunk*out of the diff.
 

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its really hard to say without being able to hear the clunking noise, but i did hear shawn's diff when it was going (which is probably the one you read about your search). basically, if the teeth really start breaking, you'll know it because the diff will start locking and lots of bad noises will be emitting from the diff. that isn't what you have, at least for now. with shawn's diff, it started making a clunking type sound when there was no load on the diff (i.e. light decel). we think what had happened is part of a tooth broke off and was rolling around in there. we even had checked the diff by jacking up the rear and rotating the wheels, but that showed no problem with the diff. so shawn decided to drive until it broke, which took a couple of months. hopefully, this isn't what's going on with your car. where does it sound like the noise is coming from? is the noise clearly rotational (does it increase with frequency as speed increases)? or is it just a single clunk when you shift? when in the shifting process does it occur?
 

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Discussion Starter #4 (Edited)
I just got back from the Subaru dealer. I had a mechanic go for a test run with me to check it out.

It turns out that the noise IS normal (like you menioned, Boy). He said that it's the viscous chatching.

I was hoping that it was something like this. I read somewhere that the AWD makes noises like this. He also explained what 10th warrior said...if the differential was really going "I'd know"...was his exact words. shuddering, locking, noises all around etc etc..

Thanks for you responses, you guys. It's good to know I have a place to ask these questions as I get to know my first Impreza.
 

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Normality

While I do believe that this is 'normal'. It's probably just a little slack in the drivetrain making itself known. You should NEVER believe a dealer that says 'that's normal'. Flying pink monkeys could come flying out of my hoodscoop everytime I used the left turn signal, and the dealer would tell me 'that's normal'. So far I've been made aware that fuel leaks, and lack of brakes are normal. Oh, and the grounding mod will mess up my ECU. And one other thing, the viscous LSD's don't 'engage' like your dealer told you, they're always 'engaged'. The only way you'll notice their action is when you have wheelspin, you can feel the traction come back, but it's a smooth transition, no clunking.
 

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Flying pink monkeys could come flying out of my hoodscoop everytime I used the left turn signal, and the dealer would tell me 'that's normal'. So far I've been made aware that fuel leaks, and lack of brakes are normal.
but that's only because you have a WRX :p seriously though, that's a good warning.
 

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Discussion Starter #8
I still have doubts and I don't like it. I was driving my RS around tonight specifically to hear this noise. How can this be normal? Can anyone explain the mechanics of it for me?

The clunk happens directly after I disengage the clutch after shifting and I start to excellerate. For example...excellerate to desired rpm..engage clutch...shift to 2nd gear...disengage clutch and start to excellerate again...CLUNK...excellerate to desired rpm...engage clutch...shift to 3rd geat...disengage clutch and start to excellerate...CLUNK...etc etc...

1st to 2nd...clunk, 2nd to 3rd...clunk, 3rd to 4th...small clunk (almost unnoticable), 4th to 5th perfect.

There is no wheel spinning at all. I do not drive hard and there are no other noises whatsoever. I can feather the clutch to prevent the clunk, but then the clutch will need to be replaced within a few years.

If it is slack in the drivetrain, how can I fix this?

Other than this (and the whistling mirror) this 99RS is absolutely tight. It handles and rides like a brand new car. In fact it even smells brand new.

If this clunk is normal, then I can deal with it...no problem. But if it isn't...then it will irritate me until I have a solution.
 

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Keep in mind that I'm not telling you that nothing is wrong. If you've still got plenty of warranty left, my advice for you is just ignore it, and wait until something breaks. If it hasn't broken by the time you warranty is up, then it's probably not that bad anyway.
To see if it's slack:
Raise your car securely on jackstands, put the car in neutral, grab the driveshaft and twist it back and fourth with some enthusiasm. See if this is the clunk you're hearing. If this is the noise, then these are the mechanics:
When you disconnect the tranny using the clutch, the back wheels are now driving the transmission. The 'back' side of the teeth on the ring gear are pushing on the 'back' side of the pinion gear teeth. Same thing is happening with the gears in the transmission. Now gears don't technically touch each other. There's a required tolerance left in the system for oil to flow between the gears. When new these tolerances are just a little too tight and will cause some excessive wear at the contact patches on the teeth. If you were to open up the tranny or diff you would notice what should be a nice uniform polished circle right where the gears make 'contact'. Someone familiar with gear setup can look at the patches and let you know if the gears are meshing properly. There's also some freedom of movement within the U-Joints. You may not hear this by twisting the driveshaft as you're probably not strong enough to overcome the padding effect of the thick grease around the needle bearings. As you engage the clutch and apply power to the system, the gears move from the teeth meshing on the 'backside' to the 'frontside'. It's only about 1 or 2 degrees of slack overall, but enough to make a noise. There's a couple ways to get rid of this noise. One is to use a heavier gear oil, it will pad the gear engagement. It will also heat up more, and reduce your mileage. Manufacturers also add tuned weights and rubber insulators to driveshafts and drivetrain mounts to absorb the clunk before you hear it. Crawl under a 'luxury' vehice and I bet you'll find about 50lbs of steel chunks with rubber mounts bolted all over the place with no apparent function.
This clunk is one of the reasons that clutch drops are bad. The system will gain momentum while taking up the slack. The sudden shock when the slack is gone can contain double the torque that your engine is producing in that instant just from it's momentum. Slipping the clutch a little before engagement removes the slack gently.
I notice that I get the noise when I shift wrong. If the RPM's are too low for the next gear, or if I release the clutch too fast, the clunk will be much louder. I have this noise in my WRX, Shawn had it in his RS, and frankly I'm glad they didn't add a bunch of sound deadening junk just to hide some NVH.
 
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