noisy rearend

[hwt03]

Hi all, I'm new here!! Just bought a 2004 Thunderbird with metallic merlot paint and only 4200 miles. I love it . I have both tops, heated seats and tan convertible top.

I have one question and concern. At about 40 MPH, if I accelerate, I get a whiring, whining noisy from the back of the car. It sounds like it is coming from the rearend. It goes away when I take my foot off the gas. Is it the gears in the rearend? Or low on oil? I bought it from a Ford dealer and they say they did a 200 point inspection on it. The dealer is 100 miles away and don't really want to drive back there. Does anyone have any ideas or the same problem? Any info would be helpful.

Thanks Harold