Thread: FIXED: "Flutter" sound only under boost. Ideas? - 2006 Grand Prix GT

Just started all of a sudden 2 days ago. When driving, braking, or accelerating (without boost), everything sounds normal. As soon as I step on it enough to get some supercharger boost (doesn't matter if it is a couple or all of the "boost" bars on the DIC) I get what I can best describe as a "flutter" sound coming from the front of the car. The best way to describe it is like a plastic bag flapping quickly in the wind, but louder. My wife says that it sounds somewhat like when a person tries to imitate a cat purring with their tongue, but whatever...

Any thoughts? Performance *seems* to be normal.

Thanks!

UPDATE: There are videos in this thread showing the sound, and it ended up being a large exhaust leak at the flex pipe on the downpipe/catalytic converter.

I would look into the air box if its stock maybe you have a bag stuck in there, or maybe something like the air filter is ripped.

Jeff

Yeah, I took a peek inside the air box (it is stock) and didn't see any debris at all.

Have been searching all over for this, and all I seem to find are people with bad couplers (which sound like a bunch of rocks tumbling in the supercharger) whereas this is only under boost. Speed makes no difference, and the second I let off even the smallest amount to get it out of boost (still can be on the throttle pretty good) and it stops. So odd.

Of course, since the supercharger only kicks in when actually in gear, I can't have someone sit in the car and "hit the gas" and listen under the hood as the supercharger doesn't kick in when it is in park obviously... Makes it hard to troubleshoot when I don't even have a location under the hood to look at....

ever do a tune up?

EDIT: Yes. See my post below for what's been done.

id start there, seems like its misfiring when you get into the gas.

It doesn't seem like a KR sound at all, not that "hard" of a noise. I'll connect my little Bluetooth OBDII adapter and monitor it a little tonight. No lights on the dash either.
Although, it did happen after I put a full tank of gas in it (ethanol blend 87 octane). Bad gas maybe? But it doesn't sound that harsh like it is knocking, and the power seems to still be there. Hmmm...

you cant hear kr, and i never mentioned it either, misfires are misfires. when they happen when getting into the gas its tune up time.

if you bought the car and never did one, just do it, start your own records.

and stop running 87 gas, run 93 all the time, or at least 91.

your killing your car with that cheap 87.

You are correct, you can not hear a knock being retarded, but you can many times hear another symptom when it gets bad enough (gurgling exhaust, the engine struggling to provide power, engine sounding like it wants to stall, etc from misfires), or there is a secondary issue. Under normal operations, most (all?) regular vehicles experience knock retard even under normal driving conditions, and you never hear it. But when things become problematic, the associated noises usually are different than what this sounds like. That is what I was getting at. This noise is just strange. Like I said, sounds like a plastic bag flapping (quickly) in the wind, not a metal on metal "ping" sound and with no real loss in power, it doesn't seem like the KR is much higher than normal, but maybe it is just high enough? Again, just thinking "out loud" and not disagreeing.

As for the gas, here in Iowa they somewhat recently switched from 87 regular, 89 ethanol blend, and 91 premium to 87 regular, 87 ethanol, and 91 premium, skipping the 89 altogether. My old GTP said premium only, the 06 GT does not, but I bet that 87 is just too low and if I was a betting man, I would bet that the manual even says that 89 regular is OK. Since I can't get 89, then I guess I will have to pay the $0.70 gallon "premium" price for the premium gas. Do you think that a bottle of octane boost would be beneficial for this tank of gas?

Octane boost is worthless. Just take it easy on the throttle and fill with premium when it gets low. Also, tune up, not a tune. He's talking about doing stuff like plugs, wires, fuel filter, engine oil, trans fluid, pcv valve, etc. Just so you know where you're at with the car maintenance wise.

Sounds good. I will save the $10 on the octane boost.

Yeah, I misread that on my phone. Saw "tune" and I think that "up" was off the screen. As for the car's condition mechanically, I've had it for a few years and do most maintenance myself. Let's see, in the last year, new quality plugs and wires in April of this year, engine oil (always has been Mobile 1 full synthetic) changed 4 weeks ago, transmission was completely flushed and filled almost exactly a year ago when I had the transmission rebuilt. Air filter is still very clean (don't recall when I changed it, but it is really clean). Supercharger oil was done probably 18 months ago. Have not done a fuel filter or PCV on it. Other items I've repaired/replaced are not related (wheel bearing, new brakes and rotors all around just 2 weeks ago, etc). Car has 130,000 miles on it. I got it when it had 85,000 back at the end of 2010.

Sounds like you're all caught up on that, depending on what kind of plugs you used, it could be time to change them out. Copper ones, usually only get a year or so out of them. Lets see what happens when it gets some premium in it, though. Being '06, I don't believe you have a fuel filter, so disregard that.

I'll have to pull one of the plugs and check. I know that with my 2000 GTP I went copper and changed them on a "regular" basis as I was all into (slowly) modifying that car. This one being stock, I went with AC Delco Iridium plugs I believe (I'd have to pull one to make sure).

Also, just as a general update. We were gone most of the weekend and took the family van, so I still have just about 3/4 full tank of gas to get through before I will be able to fill up with premium.

So I finally drove enough to get down to about 1/8 tank of the 87 octane ethanol... Filled it up with the 91 premium (best available in my area, no 93), and still drove carefully for about 30 miles. It STILL makes the noise when the supercharger kicks in.

The "rate" or speed of the sound seems to be what I would consider consistent with one of the cylinders of the v6 firing. As RPMs rise and fall, so does the speed of the "flutter" but again, ONLY when the supercharger starts measuring boost on the DIC. If I play with the throttle just right I can get high RPMs going without boost and hear no sound vs. lower RPMs with boost and then the sound is there. Of course, high RPM and boost the sound is there too. Just trying to note that RPM isn't the factor, but the boost from the supercharger is, whether it is higher or lower RPM.

Temped to put the whole front end up on jack stands this weekend, having my wife get in the car, have her put it in drive and with the hood popped put it into drive and hit the gas enough for the supercharger to kick in and then see if I can localize the sound under the hood. The problem is that this idea seems fairly dangerous. I could do it in the driveway to prevent the car from possibly coming off of the jack stands and driving through the back of the garage, but nothing would prevent me from getting run over if something like that happened, even in the driveway. I guess some thick, long, and wide wood blocks could be more stable than the jack stands, but still, there is always a risk.

The reason for needing to so something like that is just because the supercharger does not kick in when the car is in neutral and thus the sound never happens.

Can't imagine the cost of a time slot at a dyno machine just to diagnose a noise under the hood, plus I don't know where I would find that here in the middle of Iowa. At that point, I would just take it to a mechanic that can lift the whole car off the ground with a car lift.

did you try to post a vid? i forget. if not that may help.

I can try an "in-car" video to get the sound, but I am not sure how well it will sound. Worth a shot I guess. If I had a Go-Pro, I would put it under the hood and drive the car. LOL

zip tie the phone under the hood lol

I tried a few times to get a video just so that people could hear the sound. I tried with my cell phone, I tried with our digital camera, I even tried with our HD camcorder. None of them would pick up the sound over general road/engine noise. My next step was trying to run an actual microphone under the hood, but that didn't work either.

I mean, I can kind of hear the sound, but I know what I am listening for, and it really doesn't replicate the "real world" sound that I am hearing, so posting it wouldn't really make sense.

I think that I have to, unfortunately, make an appointment to take it in somewhere that can dig into it more and hear it for themselves.

I finally got some reasonable audio on 3 video clips (took these on the way to work this morning). Simple solution was to just remove my phone from its protective case.
Probably the most accurate sounding is #3. These are all pretty short videos. car is at the shop that I usually go to when it is something that I don't want to deal with myself, or something that needs more diagnostics that I can do. They already told me that if it is a supercharger specific issue that they will have to send me elsewhere. Well, since it happens when the supercharger kicks in, it could mean a visit to the (gasp) dealership.

Video 1 - This one sounds more like exhaust noise in the video, and isn't very accurate, but I included it anyway)