Dear Tom,  I expect you'll get lots of replies, all asking the same
initial questions:  Which model Orchestrelle, since solo models are
more complicated, and does the note stay on under all conditions,
or just when the roll player is on?  Do _all_ stops cause the note
to stay on or just _one_ stop?

Answers to these questions will indicate where I would suggest you
look.  That said, your most likely cause is a leaking (primary) valve,
or a tight pouch causing the valve to open all the time.  Locate the
valves and compare with the one next to it for a start.

Good luck,

Paul Morris