I have a 2004 Honda Accord 4 cyl. automatic that has 172,000 miles on it. When I am accelerating thru about 60 MPH the car starts to vibrate. While driving at a steady 60 MPH there is no vibration unless I start going up a hill that puts a load on the engine. The more load on the engine at this speed the greater the vibration. The vibration slowly decreases at faster speeds. What would create this condition? I just had 4 new tires put on the car with no change.