I've had an annoying uncomfortable 'tightness' in my right knee when riding

Hi Gary,
I have a question - intermittently for several years now and for periods lasting several months, I've had an annoying uncomfortable 'tightness' in my right knee when riding. When I used to try and mix riding with running I don't recall any tightness but perhaps after moving increasingly further from my workspace I could only ride may have been a factor? For several years I was riding 5-15,000 k's virtually all on a rigid MB with a heavy duty bike rack carrying everything from chairs to 5-metre lengths of timber! (I always found it hugely amusing myself to see how much I was able to carry. Perhaps I should have got out more!) I felt a little panicked that I might not be loading my knees enough to keep them in shape or they were 'wearing down' in less robust areas of cartilage, my cycling action was too careless or something but there seemed little alternative.....