I have a vibration/resonance that feels like it's coming from the engine or the trans around 70-75mph.  It seems to do it in both 4th or 5th gear.  This is on a 07 GT stock auto.  I bought the car with about 3500 miles and it now has about 15k.  I had the tires balanced shortly after I got it, to get rid of a wheel that is badly out of balance.  The dealership said they balanced all of the wheels at that time, but for some reason it's still there.  I want to either get the wheels balanced or have the dealership look at it again.  The dealership will obviously balk and try to get out of it, dunno if I want that headache, or dropping $65+ for a wheel balance that only goes up to 60mph...  Need some advice and thanks in advance.  

I have the same problem with a 07 GT auto after buying the car new in summer 2007.  The vibration occurs around 73 MPH and behaves like a structural resonance.  The dealer checked the balance on the wheels, but it did not fix the problem.  I think it is in the drivetrain system.  Anybody else having this problem?  

This all sounds very familiar. I have a 2007 V6 car with the 5 speed manual.  I get this loud resonance that starts to build at about 71 and gets really bad about 73 and then tapers off until it almost disappears at 75.  It has been there since the car was new.  I took it to the dealer today and complained about it.  They allegedly had three techs who drove it on the freeway at speeds up to 80 and none of them reported they could hear or feel anything.  I don't think it's the transmission or the particular gear.  I have tried taking it from 5th to 4th at that speed.  And I tried taking it out of gear and coasting with the clutch disengaged.  No difference at all.  Any thoughts?  At this point, I was thinking about having the tires looked at and balanced, but doesn't sound like that will help at all.  

Good news, finally found the problem!  The drive shaft was out of balance.  I have to give credit to my dealer (John Lance Ford) - they stuck with it until they found and fixed the problem.  First they tried replacing an exhaust bracket.  Then they sent the drive shaft out to Henderson for balance.  I can still feel some vibration at 73 MPH, but it is much better.  Hope this post helps others with a similar problem.