Its dying. Had that twice in the past with older smaller IDE drives and both died after a few weeks.
Same if you start getting repeated errors or bootup failures. DOS (and Windows 9.x) will not tell you that it is having increasing difficulty in finding a sector or sectors until it REALLY can't find it.
Clone your drive NOW to a new one [while you still can - you've probably only got a few boots left] and retire the noisy one (see some fairly recent posts on this forum).