My 2001 Explorer XLT V-6, with 133K miles, has developed quite a rumble when I get to around 60 MPH which gets worse as I increase the speed. (The condition probably exists a bit at lower speeds, just less noticeable.) Its kinda of a rumble/loud hum, and includes some degree of vibration in the floor. I put it on ramps today and looked for anything obviously loose or broken, shook a few exhaust parts, and did not really find anything. Any suggestions ? Check tire balancing ? U-joints ? (they really dont make a 'clunk' sound when I put in in gear.) Thanks.