Hi,
I have had 2 Dyson DC03's with clutch problems. The first one has a problem with the knob jumping from the off to on position after a few months. Dyson changed the clutch on this machine due to this issue. I noticed that over time the belts became stretched as when I vacuumed a thick rug the brush would slow right down and stop occasionally but not ratchet noise. I had another DC03 which had a slipping clutch on it so Dyson picked it up and changed the clutch on this and when it came back, the clutch would just randomly make the ratchet sound and also when releasing the machine from upright to reclined the ratchet would sound for around 2 - 3 seconds. Dyson changed it again and luckily it was fine after that but the ratchet alert stopped after 2 years or so, I guess down to streched belts. I also noticed fine dust & hair would make its way into the belt and pulley area so this probably didn't help.
I purchased a DC04 which had a 'noisy clutch' when it was vacuuming so I called Dyson 1 month later and they came out and replaced the clutch, this was back in 2002. The DC04 lasted until 2009. The clutch worked fine but the belts became stretched as whenever I got the brush jammed, the ratchet noise would no longer come on so I had to be alert and remove the obstruction quickly.
I now have a DC24 which I have had for over 2 years and its much more substantial with its toothed belt system. I have not had a problem with it apart from the wand plastic breaking, Dyson changed this free of charge. I find the cleaning performance to be good on carpet and hard floor but the hose suction is a little weak but I get on well with it and its more than adequate for stair and cleaning bed etc. If I need to do the car then I just use my Sebo Felix for that.