I have been getting a strange noise when I am accelerating from about 40 mph.... if I am on the highway and need to accelerate on an incline or to pass I get a noise somewhat like a rattle but not quite... sounds as if it is something turning or spinning but it does not increase when the speed increases... it goes away the second I let off the gas. sounds as though it is coming from the drivers front side. almost sounds like a bearing racing.... any ideas ??

 

sounds like it is downshifting.....