Ask yourself this question. "Why does the noise come on at 15 and go away at 25". Is it because the transmision shifted into 2nd at 15 and 3rd at 25, then you likely have a transmission problem.

If you still have the driveshaft out start the car put it in gear and rev the engine SLOWLY from 0-40 mph see if there is any noise. This will require very little throttle without a drive shaft. Also note the RPM vibration occures at. DO NOT PUT THE CAR IN PARK UNTIL AFTER YOU SHUT OFF THE ENGINE or you will hear the load noise of a parking pawl trying to engadeg into a rotating parking plate. If you get vibration doing this test you have pretty much isolated the problem to the transmission. If you dont get vibration you have not yet eliminated the transmission.

If you already have the drive shaft in the car try a road test using full throttle acceleration and followed by another using very light acceleration. This will cause the transmission to shift at different points. If the problem occures at the same speed all the time no matter what gear your in you have now isolated the problem to be something that follows wheel speed only. That would include somewhere from the output shaft of the transmision (unlikely) to the tires (somewhat likely, more likely if the problem occered at about 50 mph).

Problems that are agrevated with speed and keep getting worse as speed increase are generally accociated with pure imbalance. Problems that get aggravated within a certain speed range are generally accociated with system stability issues and may be highly aggrevated with imbalance. It is unlikely the problem is in your wheels because the typical wheel and tire combination has dynamic stability issues in the 50 mph range. You appear to be well below that speed. But if the radail plys break down in the tire side walls the dynamics could change.

The drive shaft on the other hand should have stability issues at a speed faster than the car can go unless something happened. Something that would change the drive shaft stability would be its mounting or a crack, and infact the transmission, engine and differential mounts could also come into play. The most likely components supporting the drive shaft that have been known to degrade are the drivechaft center bearing support or the rubber flex couplings.

You may want to put the car up on jack stands and run it. If the problem will occure on the stands take a look at the wheels to see if they are obviously bouncing up and down. If not remove the wheels and install 3 standard bolts on each wheel to hold the rotors firmly in place. Run it again and see if you still have a problem.

I will give you what I consider my most likely guess. And that is that your problem is most likely in the driveshaft center bearing support.

John Roncallo