Grinding noise in cockpit floor

[r_bond]

When we take Trinity (1993 walk-thru transom) out in seas more than 2ft. we hear a grinding (for lack of more accurate description) noise directly under floor in the cockpit.  Sounds/feels like it occurs between the pedestal and rudder access port.  It is most noticeable when the boat comes "up" from a wave trough.  I've checked rudder post - below decks as well as at top of rudder post via access port and everything looks fine - i.e. no unusual movement or play.  No water ingress around rudder post.  It's got me stumped.  Ideas/suggestions on what else I could check greatly appreciated. 

[Dave Spencer]

Could be your prop turning.  Try placing the transmission in reverse.  Let us know how you make out.

[Jim Hardesty]

I had a noise something like that.  I took off the emergency rudder cover and washed/flushed out the rudder top with a hose.  Noise hasn't returned.
Jim