Rinnai long fan run time after shutdown

[AkKeith]

Installed a Rinnai RE199iP about a year ago. Lately the unit sounds louder on start up for a few seconds then quiets down. the fan also stays on longer after shut down even if the unit is on for a short time. Usually the fan runs for a few seconds when unit fires for a short while. No error codes. Through wall exhaust is clear. Water still heats fine with nice blue flames. Any ideas what it could be? Thanks for any help