Socko

Hi guys, looking for some help/advise on where to look for a fun noise I have on the track. I have an 03 z06 with 34k on it, a good alignment and michlen ps2 tires. I ran Blackhawk farms raceway yesterday for the first time and was noticing what i can only describe as a thunk coming from behind me when in transition. It sounded like maybe a 6" long piece of 2x4 floating around in the trunk, and sounded off at times when anything in the trunk would have been flying around. I checked the trunk even looked under the carpet as much as i could and there was definately nothing loose back there.

It was my first time at a very small track with some very tight areas. I got significantly faster throughout the day and by about the 4th session I started to notice a noise coming from the back of the car. It seemed like when i would very quickly transition it with the chassis not completely settled, and definatly a bit more violently than i should, is when i would hear it the most often. I do believe it occured at times when the car was settling out of a faster corner and power was simultaneously being applied rapidly then i was quickly transitioning straight from power to clutch enguaged braking. It was not staticly repeatable. I was somewhat concerned and took a look under the car between the 4th and 5th run and couldn't see any super obvious wear/rubs, but of course had less time than was required for a complete inspection.

In my 5th and last run of the day, I worked on being smoother and ran my fastest lap and did not hear the clunk. I didn't hear it driving 80 miles home either.

My primary concern is i have a track day in early June at autobahncc which is more like 130 miles from Milwaukee and I don't really want to have to deal with having the car towed home cause a mount is bad or sketchy, which is my best guess at the moment. That is of course assuming that its not just normal that the rear end can move around a bit in track conditions and should be addressed (possibly just by having more driving skill) but isn't a sign of imminent failure.

Thanks for any ideas you can provide. I plan to have the car up in the air most of the weekend to swap brakes and address this issue.

RX-Ben

The sway bar end link may be getting hung up on the tie rod.

argonaut

Perhaps its the same issue - at Thunderbolt the final turn is a high speed right hander onto a long front straight. About 50% of the time coming out of that corner when I'm hard on the gas I will hear a loud thunk just like you describe. Its similar to if you had run over something and kicked it up against the trunk.

A year or so ago there was a thread about this (wish I had a link to it). There seemed to be quite a few guys with a similar thing. There was speculation it has to do with the axle where it enters the differential. You know how when you take the rear end apart, the axles basically just pop out of the diff with a strong pull? Well the theory was thats whats happening on track - the axle starts pulling out a bit and then snaps back. If there is any validity to that I can't say but it was an interesting read. It was reassuring to know (or at least think to myself) that other guys have the same issue and its not just my car.

Edit - I've tested the sway bar idea on my car. I completely disconnected it. Same thunk.

braknl8

Also, while your car is up on the lift take the load off the suspension and check to be sure your top shock nut isn't backing off. I've had this same "thud" from both the shock and sway bar endlink.

Socko

I went out and looked at the endlinks, no wear i could see. I can't find a picture online of what part of the end link would hit the tie rod so I didn't know where on the end link to look. The tie rod doesn't seem to have any scuffs or anything in it in the plane where the end link could hit it.


On a side note the car is all stock except it has kooks long tubes and the 3" X pipe to the stock axle back. And of course track pads.


I will check the shock tops this weekend, I did actually think that is what the noise could be but couldn't figure how they could get loose so dismissed the though. Apparently it is worth a check, thanks.

Interesting note about the half shafts popping out. It does very much sound like something popping up and hitting the outside of the plastic trunk liner. You are right, it helps to know that i am not the only one!

crimlwC6

Mine did that and it was the sway bar.

redtopz

Yes, often times the sway bar binds in the bushings and when loaded enough it pops/slides sideways.

jcsperson

For future reference on my part: will some kind of lube prevent/correct this?

SunnydayDILYSI

Mine does the same fairly consistently as the car unloads (02 Z06).

BEZ06

I've got a C6 Z06 and get the same clunk you're talking about quite often - mostly when accelerating out of a right hand turn.

Take a listen at 0:27 into this video and see if it sounds something like yours - you can see me look around into the cargo compartment to see if something was bouncing around back there:




This was in Dec 2008. I don't remember hearing it before then, but since then I've heard it quite a number of times. I just did an event at Daytona again a couple weeks ago and got the clunk coming out of that same turn a couple times.

I've looked all around under the car and can't find anything that looks like it might be hitting something else, so I've just quit worrying about it.

Bob

Last C5

You will continue to hear those sounds every time you track the car. The race rubber from your car and all the other cars out there gets thrown against the plastic fender wells and goes clunk!

Socko

Sounded just like that, yep. I still am going to give it a good once over, but at least i don't feel like i need to cancel track days cause of some imminant doom now. Thanks guys.

rustyguns

I have had that same noise in 2 places...Sway bar and loose A arm, damn alignment cam bolt came loose!
oh... and the sway bar bolt was fairly snug, just not tight ! :O

But no harm no foul G/L

Socko

So I was on the track today and got to witness this noise a number of times and now have some much better info on what is happening.
There is definately noise coming from the trans/diff area.It can be really loud or somewhat quiet relative to how i provoke it but it always happens the same time. What I have to do to make it happen is brake in gear, it happened in 3rd but that's the primary track gear for where i am. So i am at 6000ish rpm in 3rd. Brake down to 3500ish hard. Then when i come out of brake i hear a big bang in the back. Harder i brake the harder the bang is. If I was in traffic and was braking slowly I wouldn't hear it or would hardly hear it. If the clutch was in when i braked i couldn't get the car to make any noise. My assumption is the diff mount is failing or has failed, and the torque applied to the rear end by the rear tires braking and the motor is causing the rear end to move out of square then smash back down into position. I am wondering how I can test it? How does a bad diff mount look?

Thanks again guys!

rustyguns

put a big crow bar in there and see if it pries apart

car 99

Check the axle nuts for TQ.

SouthernSon

Same with mine. I used some of that sticky grease that comes with poly bushings to lube it up last time I changed bars. Seems to quieten it down.

Socko

So I have been trying to replicate the noise on the road and it just doesn't seem possible. But just for the heck of it I pulled the sway bar bushings off to find that all the "paint" on the sway bar was completely stripped to bare metal under one of the bushings and about 50% stripped under the other. I tried to clean the second one down as good as I could and then picked the encrusted "paint" (maybe its powder coating actually its quite thick) out of the bushings and lubed them all up.

Thanks for the tip guys, I have to assume as beat up as they were there is a good chance that was the issue.

The diff does move a substantial amount if I put a jack on it I can get it to move about 3/8-1/2" without lifting the car off the ground. The rear end on the car feels so disconnected and floaty anyways I figured a pfadt trans mount was a good place to start, so one is in transit to me now. Between it and greasing the bushings I suspect this issue should be resolved.

Snuckley

Just to help others with thunking C5's, mine was caused by my custom straights pipes hoops hitting the the tranny tunnel. But only when I was climbing the steep hill on the road I reside on and would hit a hump in the road maximally compressing the suspension. Never heard it on the track.