Hey guys, hoping somebody on this forum can help me out with a problem. I've done a lot of searching but can't find anyone with my specific problem. I have an automatic 2007 Z with about 45k miles on it.

It doesn't get much highway mileage but about a month ago when traveling out of state everytime it would reach exactly 70mph a noise I can only describe as helicopter thumping starts under what sounds like my passenger floorboard. If I slow down to 69mph it stops. It does not appear to get faster if I speed up about 70mph and there is no vibration at all.

Most of the driving I do is city and there is no noise at all, the car keeps straight and has no other problems.

An SUV backed into me a few months ago and the bumper and some front end components were replaced, at first I though the shroud under the engine was flapping but it appears to be secured.

Anyone have any ideas? Any help would be greatly appreaciated as I'm going out of state to visit family next week.

 

That could be a defective tire. I had the same problem you describe (not on my Z but on my Jeep). I couldn't see anything, but the dealer determined the tire layers were separating. Replacing the tire fixed the problem.

 

Which front end components were replaced? Please be a little more specific. Did the noise start right after the repair? If not, when did you first notice it?

 

I got it on a lift today it appears the suspension is sound. My tires have noticable tread loss on the inside but what appears to be the problem is that when the dealership replaced my bumper they did a crap job of putting on the under engine shroud. It was drooping and whatever the plastic piece directly behind the shroud (maybe an extension of it) is broken on one corner. I attatched some washers and tried to straighten it up. I'll be on the interstate tomorrow so hopefully the noise is gone. I will keep this thread updated for future searches.

 

My guess is that it has to do with the tires you're running.

 

^^ @VMRWheels, I agree. The OP is pursuing a solution that repairs some loose shrouding, but this is not the cause of the problem he reports.

The OP said:
Exactly at 70 mph!!?? …That can only be tires.

--Spike

 

Update and fixed itself. It was the extension of the shrouding (a plastic square behind the shroud under the engine). After having my car on the lift and checking the mechanical components I thought could be the problem I felt as though it was safe to drive at 70mph, when I first noticed it I slowed down to avoid the noise. About 2 hours into the drive the noise changed to a barely audible tapping, after a minute of listening to it there was a loud pop and when I looked in my rear view I saw that it was the piece of plastic popping off.

My best guess is that 70mph and up winds were just the sweet spot of force it took to move the piece.