I've had my SD60 for a bit over three years and it still only has 27 hours on it. I'm gone a lot and someone else mows when I'm gone. However, since the mower was new, it does almost kill the engine when engaging the blades. It shakes hard, makes loud rattling sounds and then calms down as quickly as it started and I then run it wide open both in engine RPM and land speed.
I'd say it is because the decks are floating so loosely the belts grab and almost suck it backward that makes it sound so loud.
Check for loose bolts, motor mount, etc anyway.