(ET) Diagnosing an extremely noisy mowing deck?

[Charlie <medievalist gmail com>]

Hi guys

My front mount deck vibrates like the Tacoma Narrows bridge, and
balancing the blades has not helped.  The constant rattling is very
annoying, and I've had to switch to cotter pins above the front mower
wheels because the stock pins shake right off.  Is there an easy way
to check for a bent shaft without disassembling the entire shebang?
Everything feels solid if you grab the blades, no wiggles or wobbles,
and when the deck is flipped up vertically everything works perfectly
(sounds like a big smooth-running electric fan).  Once you start
mowing, though, it rattles and bangs constantly.

--Charlie