Hi, 

On a certain stretch of road going down hill around 60mph and 3000rpm my engine or maybe transmission makes kind of but not exactly whirring noise. The best I can come up with is the transmission downshifts for  automatic braking is making the noise. this has happened at least 2 times . recently Before this started happening  my D light was blinking and I had a transmission pressure sensor replaced to fix that.

My mechanic I have been using for years  is very dismissive about the whole thing I had him listen to a recording of the noise but he  still he doesn't know what it  is , he says he needs to hear it for himself , but it only happens in certain conditions it's unlikely for that to happen. I had him look things over but  he says there is nothing, but I think he just gave it a once over just to get me of his back and didn't look too hard. I'm concerned there is low transmission fluid.

My car has 56,000 miles on it.