91 LS400 Extreme Loud Noise at 65MPH or more.
 

Hi, I have a 91 LS400 which runs great and quietly (most of the time) except when the vehicle is running at 65 mph and above. The noise comes from the engine compartment and it is continuous. It happens like clock work. It is much louder than the car's horns and it scared me the first time! The noise is like an air horn but slightly lower pitch. Trust me, I don't have an air horn. People always look at me as if I am honking at them. However it does not affect the car's performance.

To avoid the noise, I set the cruise control at about 64 (or slow down) then the car runs like a kitten.

Can anyone share their experience? Thanks.
Alfred Li

I tried to upload a video clip but apparently the file was too big; therefore I posted it on Youtube instead. Below is the link to the video on Youtube. Hope you can see it.
Today is a stormy day San Francisco and the sound appears earlier. I think it may be related to the wind but I looked under the hood and under the car and everything seems to be secured. Please advise!

Wish we could hear it. So what is the RPM when you hear it? If you sit not in gear and rev the engine to that RPM does it make the noise?

yea, can you be more descriptive?

Maybe that is the magic speed where air flow is pulling the not fully attached plastic under cover down and it either drags on the road or just flaps like mad. You need to determine if it is the engine or the car. Go 68 and put it in neutral and let the engine idle and see if the sound is still there.

I agree, it sounds aerodynamic. Maybe a safer way to do the test dicer is suggesting is to drive uphill at 68 and then coast downhill at 68 and see if the sound is the same or different.

But it's easy enough to first take a quick look under the engine to see if it is loose.

Thanks for the suggestion. I will take a video so that you can hear that noise. If a picture is worth a thousand words, a video must worth a whole lot more. Give me a day or so. Thank you for your patience.

And in the mean time look under the car in the front and see if the plastic belly cover is loose, like oldskewl said.