You have a ground loop set up causing a feedback hum.
something is not plugged in all the way, or something that was not
plugged all the way in, is now.
A ground loop can be caused by attaching a ground as well as by
detaching one.
A ground lead from your stereo to your turntable chassis may stop it.
If it does not, then you need to 'free' a ground which is attached on
both ends of a cable, at one end.
Your "RCA" connectors can get oxidized on the outside. These are the
jacks you plug into with your turntable leads. They either need to be
better attached, or 'less' attached.
I know it is confusing, but a loop can be created by making a ground
connection which should not be there, as well as by not making one that
should be in place.
If you recently hooked the turntable up, you should re-check those
connections carefully.