I have an '02 Accent with 184,000 miles on it, and I currently drive 800 highway miles a week. It's been an extremely reliable car.

Just this week I have started noticing an intermittent vibration at highway speeds. It seems to happen between 65-72 mph. At first I thought it was a bad spot on the highway, but I've noticed it on different sections of the road now.

The vibration doesn't seem to be coming from any of the tires, but rather feels like it's right under my feet. If I speed up or slow down, it usually goes away, but will return. My tires are 35,000 mile tires and have about 20K on them. I rotate them regularly and check pressure weekly. I replaced the right front wheel bearing at about 160,000 miles, but this vibration is totally different from a wheel bearing vibration. Again, it feels "centered" in the engine compartment/firewall/floorboard. Thoughts?