Help with clunking noise

For the past 4 days my 1998 Plymouth Voyager has been making a clunking sound almost directly under the floor near the brake pedal. It only clunks when I am traveling straight down the highway, stops immediately if I turn even the slightest bit to the right or left and slows down as my vehicle slows down. There is no shaking or shimmying of the steering wheel but I can feel the vibration of it if I put my foot on the floorboard as the van is traveling on the highway. Any ideas on where to start looking? I can't see anything hanging or loose. Thanks for your help!

 

Could be bad wheel bearings. they do make clunking noises when going out, but usually only when turning.

Inspect your CV joints. Especially the rubber boot, if this is torn off, the CV joint will eventually go bad.

 

CV's clatter on turns ...........Bearings usually hum.........I'd be checking my tires for flat spots/cupping..............and if my car had about 100,000 on it in 9 years I'b be thinking about a pair of struts and an alignment