Vibrations @55-60mph

Have a problem with my 99 olds intrigue GLS,ther’s a vibration problem @ 55-60 mph the serverity varies(from almost none to intire car shaking),I check and rechecked the tires, front end alignment(replaced tirods ends) ,wheels balance and tire pressure-could it be an engine or Trans.problem? (it sounds like its missing when the car vibrates-or my imagination)) though both have been looked at but not relly checked in depth,wish I had the answer.

Is there a check engine light on?

Is it associated with anything in particular such as braking or accelerating or any other thing related to what you are doing at the time?

Elaborate on “sounds like its missing” if you can. How are your basic maintenance items? (Plugs, wires, filters…)

Uneven tire wear causes this kind of problem. Try this, rotate the front tires and see if the vibration goes away or happens at a different speed. If not you can rule out the tires.
You have a few more potential part problems that could cause a speed-specific vibration:
struts, ball joints, stabilizer bar, even motor mounts. Get under the car and inspect the condition of the front end parts, looking for excessive wear of components, ruptured bushings, that sort of thing. If you have never replaced the struts, test them by putting your body weight down on the front corner fenders and quickly release the fender. If your struts are good, the fender should rise a little higher than it’s original position and then resume the original position and stay put. If it rises and falls a few times, you need to replace the struts. In heavier, even mid-sized cars, bad struts create rather than a vibration but a sense of loss of control at specific speeds, because the car will rise and fall too high and too low after hitting a depression or rise in the road while driving, and this action will cause the front tires to lose contact with the road, causing a loss of control similar to hydroplaning or driving on ice. In smaller cars, however, bad struts might just cause a vibration.

Excessive negative camber in the back suspension will cause the tires to feather. Once there feather the tires can cause vibration and noise no matter how perfect they are balanced. Has the tires been checked for uneven wear, feathering, or cupping? If so then perhaps a four wheel alignment that puts the rear camber to neutral, and a new set of tires will fix it.

How did you check the tires? I only ask because I have a slightly imperfect tire that produces a similar vibration which looks fine and is balanced. It is worse when it is on the front. But the roads are so bad here I hardly have a chance to notice it.

If it were the engine I would suspect the vibration to be correlated with rpm and not mph.

Is this a front or rear wheel drive car? If its a front wheel drive have the CV joints and hub bearings been checked. If rear wheel drive have the universal joints checked. I assume the vibration goes away over 60mph.