Hum by ac motor with lingo3

Hello to all! I am new in this forum and decided to join you guys after reading all the info about the best TT on the planet! I have an old 1984 LP12 with corner bracing upgraded from Ittok with Troika to Ekos2, cirkus and Arkiv B in 1996.
Last month my dealer changed the top plate to fit the new one with the bolt, new ac motor and the Kore. The result? Wow!!!!!!!!!!! The kore is amazing! All parameters flew sky high!
My second TT is Thorens TD125mK2 with a Sumiko Premier mmt tonearm and Benz Ruby3 (low 0,4 mV) cartridge. Te LP12 is obviously much better.
Having high efficiency Avantgarde Duo Grosso horns I was checking last night all the grounds in my system because I discovered a strange problem with my beloved LP12 wich does not happen wit the Thorens (also with ac motor). Grounds are OK.
Turning the volume to max. no noise or hum, but moving the lifted tonearm towards the center after half way I start hearing hum from the speakers and the hum increases as I the tonearm gets closer to the center and obviously closer to the motor.
Is this normal with the ac motor? If no please help!
Please, any suggestions to solve the problem