I love my Grasshopper and I have a 2003 321D that is the best mower I have ever had. Built like a tank with 537 hrs on the clock. It has developed a weird oscillating noise with the mower engaged. Sounds kind of like a blade is hitting a shroud in the round sheet metal surrounding one of the blades. I don't see anything and it comes and goes while the mower is running. About +/- 3 second intervals. All spindles and blades are tight. None of the idlers appear to be worn or loose. Has anybody ever seen anything like this, if so what was the cause?? Don't feel comfortable waiting until something breaks or falls off! Any help appreciated. TIA