Hi Paul,
I just ran across this a few days ago. I had never heard of a stirrup plate before. It looks like it gives you a little lift, 4-5 mm, up off the plate.
http://leatherworker...showtopic=51037
I think the noise may be the hook hitting the needle. If the rolled edge of the sheath is dragging on the bed some distance from the needle it has torque/leverage and could be deflecting the needle. Try this... make a stack of leather like the sheath but without the fold so it is flat. Run it through, if it doesn't make the noise, dummy up a sheath with the fold and put something slick between the bed and the fold to reduce the friction. I'm thinking a piece of zip lock bag, slick magazine cover, Mylar, or even satin ribbon like you would use to wrap a gift. It may not eliminate the noise but, if it reduces it you will know what the problem is and then the stirrup plate could be the answer.
Best of luck,
Ray