2000 4Runner noise while maintaining speed

While driving at speeds approximately 30 mph and over, I get a chatter type noise coming from the engine area. I say 'area' because it's closest that I can come to. If I accelerate or let off the accelerator, the noise will decrease or even stop. It only seems to occur while maintaining current speed, accelerator slightly pressed with the RPM around or under 2000. It is not affected when turning the steering wheel nor while braking. The only maintainence that I've done are oil change and new tires. I've had problems with the tires and wonder if the store rep has been giving the run around. Any ideas? Regards, MDOwens

new spark plugs.. wire set, rotor cap.