What Causes Knee Pain?

going up and down stairs and going up and down from the squating postion make my knees grind to the point it will turn peoples heads but there is minimal pain if at all. this started all of a suddon with no injury about 2-3 months ago. i can walk fine with no grinding. i am over weight and working to lose it but my legs are very stronge. i can leg press 800-900lbs with ease. i have an appt. with a ortho doctor but was hoping you could give me some insight of what this might be. i work on furnaces and air conditioners for a living and i am always going up and down stairs. I also kneel on grass and cement sometime for hours at a time also when i kneel i tend to sit on my ankles witch is full bending my knees.