If it were my problem, I'd run the lathe empty and use my stethoscope to
listen for bearing rumble. You may also do the old mechanic's trick by
placing the steel end of a screwdriver on or near the bearing cap and the
large end butted up to your ear. If there's interference at all - repeating
change in pitch - then you might consider adjusting preload in accordance
with your instruction manual.

That said, I'd bet it's just plain old flex and chatter. A cross-grain bowl
presents two mutually reinforcing problems when a scraper is used. First is
the natural deformation of the bowl as you exposed and dried the damper
interior - bowl will be narrower across the face than along the long grain.
Second is flex as the bowl tries to get away from the scraper. That bit of
tearout that you see as you pick up the unsupported ends of the fibers makes
the bowl squirm away. The combination often produces chatter marks, always
produces noise.

What to do if it's natural? Change your scraper angle to get a bit more
shear, or go back to a broad nosed gouge and cut/shear down slope not
outward . In either case, don't be anxious to get the wood off. Just take
the mild cuts and keep the speed low, and the chatter, if there, will be
minimized. A bowl steady will allow you to get much thinner, but I find
somewhere around 7mm to be the point at which chatter becomes inevitable on
a 12" piece.


"Denis Marier" wrote in message
...
Thanks to everyone for their comments.
Maybe the situation would have been better depicted by using the
"Harmonic
vibration " instead of humming noise.
After about one week I have found that all the comments are valid.
The Harmonic vibration is probably produced by the unbalance in rotating
components of my lathe. This may be further compounded by resonance
caused
by shattering of the scrapper applied on the thin wall of the maple bowl.
I
was told that in some instances the rotating components may be fairly
balanced but the finishing gouge or the scrapper lightly rubbing on a bowl
could produce resonance. I was told that the same thing takes place when
rubbing the lip of a glass with your finger. Some time bowl steady are
used.
Now, If only I can find a way to adjust the bearings I may reduce the
Harmonic vibration a little?