This is difficult one to diagnose so far. I'm guessing that you don't have another Beomaster to check the Beogram with? And that you do not get a humming from the radio? And that there is humming from the record player before the arm is lowered, ie as soon as the gram is "live"? You are correct that the DIN plug from the gram should carry an earth signal already. Peter is almost certainly correct that the humming is caused by a bad earth - somewhere.
There are two gram sockets on your Beomaster - number one is for grams that already have a pre-amp and number two is for grams that do not. I think I'm right to say your Beogram 8002 does not have a pre-amp built in and that you should be using socket two on the Beomaster.
You need to hunt down where the problem is - try using a fly lead from the centre pin of a spare socket on the Beomaster to a metal part on the Beogram chassis. If that cuts the humming then that will at least confirm an earth problem. A multimeter set to measure resistance will help narrow the problem down to either the record deck or the amplifier. Chances are most likely that it is the earth connection to the centre pin on the DIN plug of the gram.
Good luck, Steve