With the speed that those blades turn I am wondering if it isn't a harmonic oscillation.
That's sort of what I was thinking, but I'm no engineer or mathematician. I thought maybe a weak spot in one of the belts builds up to a tightness and then relaxes, or something like that. The dealer was at my place once and listened briefly, but as usual with mechanical malfunctions, it was not very intense. I don't know if it can actually be heard standing beside the mower. Sitting on the seat, it's definitely noticeable, but maybe as much felt as heard. I'll get someone else to work with me to confirm what I hear, and then back to the dealer.
Mostly I wondered if anyone else had experienced anything like this on a 3300 deck, or any deck for that matter. I don't want the spindles to be damaged and find out after the warranty is out.