What is it?? Noisy and potentially terminal.
There are a number of 'quills' on the Main Xmsn. The input quill which drives onto the Main bevel gear and the No.2 Hyd quill which is driven by it. Also the rotor brake quill if you have one fitted.
Then there's the T/R quill which comes off the sump case. The No.1 Hyd pump is driven from a quill of sorts on the RH side of the sump case and the Nr tacho gen is chain driven from that so it doesn't really qualify as a quill.
They are the only quills I can think of (unless you write with a feather that is).
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If it ain't broke..........