Wheel Hum

I just got a 99 cobra and the only real problem that I see is that at speeds above 25 mph there is a hum coming from the front right wheel. It gets variably louder at higher speeds, and tapers off around 60-65 (but comes back again at 70). I have changed the tire with a different set, and the problem persists. Any ideas?

Possibly a warped rotor...do you feel any steering wheel vibration with it?

there was a huge debate in the 4.6L section that rotors do not warp

A warped rotor won't make noise. It will cause a vibration under braking. My suggestion is a wheel bearing.

Second on the wheel bearings--replace them and the races, the hum will likely go away.

No steering vibration... We had a good idea that it was the bearings, but when we pulled off the tire and grease cap, we found that it was a different bearing setup than everything else we had seen. We checked a Chiltons for a 96-98 cobra, and it had the same/similar setup there. They say that the entire assembly will have to be replaced if it is bad.

Is there a surefire way to see if it needs to be replaced?

3rd on the wheel bearing. its a sealed non serviceable unit that does occasionally go out.

Check to see if the "hum" lessens or goes away when you turn one direction or the other. If so, it's definitely a wheel bearing as others have previously mentioned. I had the same thing on my car last fall. Replaced with an Autozone hub and noise is all gone. You simply cannot tell by looking at them whether a wheel bearing is bad or not.

Yes, it's a sealed hub/bearing unit and costs around $50+ at Autozone.

Good luck....

Is this something that would require replacing both front units or does replacing just the one work fine?

One will be fine. Do it soon.

I had an 86 GT years ago and lost it in the snow--popped a curb with the right front. It wasn't enough to damage the wheel, but I had a distinct hum for several weeks exactly as you describe--above 25-30 on up and hmmmmmmmmmmmm........ Replaced the bearings and back to normal. You said you just got the car--maybe someone hit something before you got it, but I wouldn't wait to fix it.