2010 mustang had a intermittent vibration at 40-50 and again at 65 mph and a little bit of a roar with it

Tom is correct, move the front tires to the rear to see if the vibration changes. 2010 Mustang has a 2 piece drive shaft and pretty sure that is not going to be the problem.

I tried swapping the tires nothing changed, the sound is always there it's just more noticeable at 40-50 and over 65 . I checked the front bearings they seem to be fine .

The tires are Cooper weather master wsc , they look almost new ,they are 235/50/18

Those speeds indicate tire construction issues to me. try pressure up to max then down to min. road testing each, notably different ? See if your dealer can diagnose by using another set of tires from a stock unit that does not vibrate. being out of basic warranty, you may have to smile pretty and take the service manager out for drinks to get favors but the tire swap is a very good quick answer. There are other tests, measurements and remedies available in the service manual but another tire with a different rigidity to the sidewall and a different tread design may be the fast answer. See how the pressure adjusting and driving makes it sound and feel, first. I spent numerous hours with one that vibrated at 65. The engineers wanted us to add a dampener weight. It did not have a tire issue at 40ish.

Being 18's you dont have much give in the tire sidewalls. Runout tests with your rubber off can prove wheels to be out of spec. runout tests before removal can show tire irregularity.

Stock wheels?

Have the tires road force balanced. I'd bet their road force balance weights are out of spec.

I know a guy with a mustang I'll see if I can try his tires on mine and see if that stops the noise ....thanks for the help.

There's a static or dynamic balance. we can bet he has them balanced for both. please tell us what is meant by road force balance. His problem occurs before the speed of wheel balance. and then returns at 65 this is tire and wheel shape or irregularity in the tire casing alone or the wheel shape. adding weights to the wheel wont cancel its shape.

There is an electronic carry aboard meter that measures first and second order vibrations. once you determine what you feel is one or the other, you can assign tests to the shorter list of the cars components. Your dealer has it or can borrow it from a DSE. He is the engineer and teacher locally.

The thing about the car regardless of what tires are there is a harshness they were trying to dampen. without softer spring rate, all they could do is place weight to get the springs involved more......................................But what about this ROAR ? Is this there, at that speed, in neutral ? Still there with engine off at same speed? See if it eliminates with the other set is a good plan, then cancell in- gear, then engine running

A rotating part like an engine fan fighting the wind makes a roar. you can eliminate it at 65 on a hill on a freeway. you can inspect and turn by hand before the road test.

Noisy bearings rumble at their favorite speed normally 30 to 35 I can see why it's tough to tell what it is. thats why they made the measuring device. it's like a reed tach and it expresses vibration electronically once defined the vibes or sound can be put in a category taking a list of possible parts away quick........................................But I want to hear what this balance is and how it differs from spin balance. We used to spin balance on the car with a machine and a clamped-on weight back when drums and rotors were heavy. It is not that is it?

The sound does not change if you accelerate or coast or shift it in neutral and rev the motor . I put my spare on the front pass side and it did change the sound it not as loud .

Tread meting the road surface or the wind it makes at speed they call a harmonic. My goodyear LT's were notable for a harmonic sound. They lasted but complained alot. when very worn you could see irregular shape in the tire casing. Still have one that holds air. But I wonder what your tire dealer can do for you.

They try not to recall tires, but they can do what's called "adjustment" if they really try, take the time, do the paperwork, and get the rep to agree you can get a 40 shaker that returns at 65 and with a harmonic off a customers car, pay a difference and get into a different model. But they have to really want to do all that. Seen it a few times. The rep gets pissed. But the dealer decides to restock or order the minimum. They need to consider the customers point more. It's tough to find actual good people in business situations like that.

I looked into that force fuck balancer, it runs a roller against the tread to measure lateral force during the spin balance...............The guy spins an absolutely perfect wheel and tire assembly, then the machine tells him where to divide the weight for two places on each plane. reminds me of the foot pedal balancers we used in school. then mathematically divided the weight. They could not make eggs round either and it was $3.00 a wheel.

Since you had a like spare, and something changed using that? You can see clearly, sell your set and seek something that measures true. Talk to the guys at the mustang meet and see who has the tread you like that is not amplifying the cars natural harshness and body response to spring rate would be nirvana.