Grinding/Vibration at 30mph

For the last couple weeks i've had a grinding or vibration in the front end (i can feel it under my feet). I started noticing it over a month ago but it wasn't that bad. It doesn't happen when I brake, It only seems to make a noise at around 30 mph, whether im going easy on the gas or just coasting. I don't feel or hear anything when i get going faster than that. I also don't hear anything at really slow speeds or when i'm turning. I have a 2004 V6 w/ 93,000 miles. Haven't had my brakes replaced yet, but they feel the same as when it had 12,000 miles. Could this be brakes, wheel bearings, something else? appreciate your thoughts...