knocking noise on grizzly lathe

I have a grizzly g8691 lathe. It seems to work OK but at higher speeds it produces a knocking noise. I'm going to try and take the cover off this weekend if I get the time but I'm wondering what the noise could be?
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Sorry to say it is usually headstock bearings. It might be a loose pulley knocking on the key. But even if it is, by the time you get the thing apart to fix a key properly you might as well replace bearings while you're at it.
Good luck,
-- ******** Bill Pounds http://www.bill.pounds.net/woodshop

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