Not sure if I understand exactly what is rubbing what. The crankshaft pully part of it is throwing me a little, But i have the same problem with an eXmark 48". I removed the deck cover and listened so closely that I almost lost an ear. I deduced that it was the PULLY itself that made the howl while slowing down after disengagement. I thought it was the friction of the belt, but that's not the case. Now, I don't know if the pully is about to quit or what, but I got a bunch of 'em anyway. The pully howling on my machine is the one that is mounted on an "arm" and is moved into the path of the belt to engage the blades. ( I have no idea what you call it )
Hope this helps or at least eliminates this possibility.
IF YOU AIM AT NOTHING, YOU'LL HIT IT EVERY TIME. Zig Zigler