I have a vibration that I can't figure out. At certain speeds (45-50, 70-75 mph) it is very noticeable but seems to go away at other speeds. It is a WHOOM-WHOOM-WHOOM noise accompanied by a very strong vibration in the front left side. Tires/wheels were swapped front to back and the problem stayed in the front left. Shifting gears and changing air pressure doesn't affect it either. I have balanced the tires 3 times.

What could it be?

Is it more noticeable when braking? Does the vibration change with speed? Could be a warped brake rotor.

Braking doesn't affect it, and it only shows up at 45-50 and 70-75 mph. It does sound a little different at 70 mph than 45. Quicker WHOOM-WHOOMs.

The GXP is really not my area and they tend to be rather different. My car had a vibration for a few months, then I started getting a pulsing on braking. It was a warped rotor.

Turns out that the problem was being caused by a bad alignment that I had gotten done at a local shop almost a year ago. I noticed that I've been wearing the outside edge of the front tires pretty heavily over time so I took it to a different shop to get it aligned. After they aligned the wheels, the vibration went away immediately (although the irregular wear patterns are still a little noisy). He mentioned that the toe was pretty far off. The car drove perfectly straight before and after which is why i didn't think of alignment.

Glad to hear you got that resolved.

Watch out for the wheel hubs on these cars. If you start noticing vibrations while driving, these could be going bad. Mine were trash by 45k.

Thanks for coming back with the solution!

I just did a wheel hub yesterday, front right went after 101K. It was a pretty distinct sound that started at about 30MPH and got louder the faster you go.

You got lucky. Mine didn't make any noise at all, just had the front wheels vibrating at over 55mph. It was more of a shaking than a vibration as if your wheels were unbalanced, but they were balanced properly.

thanks for sharing
mine does the same at higher speed. more like 80 mph and high, but i think it has something to do with my tires, ill know next week when my rims come in

Good point, we have had a number of bad front hubs here for a newer model car. I know the vibration issue well. When my GXP sat on front struts to ride low, I would have to raise the car considerably at highways speeds to get rid of some hard vibrations. One bad thing about adjustable front ends is toe is affected and caster can change dramatically as you raise and lower.