0hsix, I have the same problem...apparently... I don't notice it, but everyone who rides in my car says it sounds like what you are describing. Its not the tires, I went from the Stockers to Nitto 555's and the sound was still there. I think it may be gear whine but I could be wrong

 

Is your tranny auto or manual? I had a completely stock 06 and the transmission whined, Ford said there was a problem but my dealer took it back and tried to give it to me which we did not accept it. Called Ford and found it the truth, got real angry and gave them the car back (lots of ongoing problems with them led to this) so we never actually found out was wrong with the tranny specifically, he knew there was something abnormal about it though.

 

It's a manual, this started around the 40k mark.

 

Mine whines as an auto...as you said its coming from the rear, at least mine is.

 

If it's not mechanical, I wonder if that sound could be coming from your stereo since you said it increases with speed... I had a bad headunit once that would cause a "whine" to come from the rear speakers as I drove... You could hear the car RPM's increase through the speakers...

Don

 

Turn up the radio...this a Mustang, they make noise, alternatively give it to me and buy a Prius!!!

 

I had considered this, did it still whine with the radio off? Also did it really increase with rpm or with speed?

Oh and to the prius comment, I can deal with rattles and buzzes from the cheap plastic dashboard, got a set of stingers to drown them out =]. But I like to pinpoint the different sounds before I write them off as nothing.

 

In the instance for me, yes, the noise did persist, even when the radio was turned off, almost like a bad ground, which it could have been too... It would increase with RPM.

Don

 

Thanks, this seems like the most reasonable cause so far although mine is dependent on speed rather than rpm. How did you fix it? Or did you just leave it be?

 

I'm with you. I like to know what has happened to cause a change. I wonder how many of these guys in the "don't worry about it; turn up your radio" crowd have ever been stranded on the side of the road because they didn't listen to their vehicles telling them that something really bad was going to happen. To each their own, I guess...

On a somewhat more topic-related note, I get my tires rotated roughly every 15k and I usually notice some cyclical sounds either just before or just after the rotation. I figured it was just tire wear (oh, and I turned the radio up :-P)