2001 Porsche 911

I have a 2001 Porsche Carrera 4
It has 38000 miles

For the past 5 or so months I have heard a scraping sound, when the car is doing 30 to 40mph under slight load with a small vibration.
This sound is coming from the front or the car
The noise goes away when you accelerate more or take your foot off the accelerator.
The noise is not present from 50mph and up, and not when slower then 30mph.

I have taken the car to a Porsche dealer In Miami, but they said they did not know what it is and suggest just drive it till something happens?
I have taken it to another Mechanic shop specializing In German cars, they thought it was the drive shaft.
So under my extended warrantee, they replaced it with no Improvement?
They said it had to be the front deferential, again replaced that and noise still there?

I plan to drive the car to Houston from Florida in the next 2 weeks; I am hoping this mysterious noise will not leave me beside the road.
Does anyone think what could cause this noise?