2006 C2s-- I have searched this forum the best of my ability, but cannot seem to find a reference to this problem: at 75mph and up, if there is a cross wind I get a constant flapping sound from the right rear side of the car like there is a body panel loose and vibrating. It really scared me when I first got the car-- I was coming down I17 with a group of cars cruising at 90-95 mph-- and at speeds up to 115 mph (the pack leader was a Ford f150 if you can believe it!!) and the noise became so unsettling that I pulled over to troubleshoot-- and lost the pack, of course.... But I found nothing loose or hanging down then or since then but I still get the sound. I have knocked on the underbody panel, and it seems to not take much to have it contact the steel body pan and make the sound, but I can see no plug or rubber isolation bumper or anything that would indicate anything is missing or broken. Have any of you gents encountered this or know where I could specifically look to investigate or resolve it??? Thanks much-- sucks to have an Autobahn capable car that maked a crappy noise at high speeds!!!

 

Check all the bolts are tight(even IF they are a bit loose will allow the panels to flap) and that the panels are put back on the car properly!The panels go together like a puzzle and if not installed properly may cause the 'Flapping' noise you mentioned.
Were you in for any service work before you noticed the noise?

Stacy

 

not good, get it checked.. had the under panel on a VW come off >100 and sounded like a blowout.. was scary.. and not ideal to the people behind me on highway.

 

Back in April of 2008, I was running my 08' C2S out on the highway at about 80 MPH on a very windy day. The car developed a loud "flapping" noise, which seemed to be on the passenger side. It sounded like I had a plastic bag or something caught on the car. I stopped, looked underneath and all around, and didn't see anything. Started out again, cruised up to 80 or so, and the loud flapping noise continued. It was really loud. Like when we used to put balloons on our bikes when we were kids. The noise stopped when I slowed down in traffic, and got into the city out of the wind. I didn't take the car back to the dealer as I figured they wouldn't have been able to duplicate the high "cross wind." It occurred only on that day of high wind, and has not happened again since.. So I never found out what might have been the cause of noise.

Probably not much help, but I thought I'd share my experience.

 

Weird: I have had the exact same thing - sounded like a plastic shopping bag was caught under the car and was flapping like crazy. I had the shop take a look when I was in for an oil change and they (dealer) said everything was tight. It happened again a while later - I noticed it was during a strong cross wind if that means anything.
I was beginning to think it was all in my head - but now reading this I will have to dig deeper.

 

I had this a couple of months ago. Put the car up on a hydraulic lift and traced the problem to the plastic under body panel that sits under the gearbox. It has a built-in air scoop that rams air up to the diff, and the panel had effectively split in two and was flapping at speeds over 70MpH. New panel (was only $75 or so) fitted in under ten minutes and the problem has gone. I did however notice that some of the other under body panels (there are four, I think) were warped and the interface between two of them wasn't sitting correctly, so i will probably have to replace another one or two of them. This is almost certainly your problem.

 

check your rear axle cv joint

 

The car started making the noise when it only had about 6K miles on it, and I had not had it serviced in any way since I bought it from the original owner at that mile mark-- and all he had done was change the oil at 500 miles-- which wouldn't account for any under body panel removal or adjustment, I would think... I will try to get under it this weekend and see what the biz iz. Maybe he ran over something (I sure haven't!!) that compromised a panel or something...

Thanks again for such quick responses!

 

There are 6 plastic underpanels, and they are organized as you can see in the photo below. They are held in by dozens of fasteners, not all the same, in fact. Some are TX bolt, some are hex bolts, and some are nuts. Go figure. They fit together rather obviously in terms of what goes where, but where they meet they interlock like a puzzle, where some tabs must go over slots, and some go under slots, and some go into slots. I created a simple way to keep track of how they go together, so that when I took them all off, it would be easy to get them back on correctly.

I used a Sharpie to write a number on each joining tab. A "1" indicated closest to the groung. A "2" meant it was above a layer to the ground. And a "3" meant it belonged above 2 layers from the ground, etc. Some joint intersections had as many as 4 layers, so it was tough.

Spending the time to label before dismantling made re-assembly a breeze.
CATTMAN

 

A long shot, but easy to check: once on the highway I also heard a flapping noise at speed. It turned out to be a plastic shopping bag that had lodged in one of the front radiators.

 

Got the car up on a lift and checked all the panels-- everything was fine and appeared to never have been previously removed from the body. All screws, nuts, and fasteners were present and tight, and no panels had been hit or damaged such that they were visibly suspect. They appeared to have been properly fastened at the factory as well.

The only thing I noticed that could have caused a noise is that in the middle of the car there was about a 1/2 gap between two of the panels. I drilled a hole in one of the car body supports and screwed a self tapping screw in to suck the bottom plastic panel direct to the support and that was all I could see to resolve any flapping-- even though many of the panels seemed to make contact with the bottom of the car if I bounced them with a knuckle, etc. Kinda a chintzy way to construct an expensive fast car if you can't make it quiet at speed (don't they have cross-winds on the Autobahn??).....

Many thanks to Cattman for the great pic of all his bottom panels!

Hope this fixed it. When you are really rolling at speed, I really pay attention to the car and any 'unusual' noises, so a flapping gets your attention real fast!!!