That's the situation with me too. They didn't replace it for me either because there is no such fault. I should go home, the fault will become more apparent later. The fault didn't become more apparent, but I kept banging on the table until the mechanic came with me. They said there was no such buzzing noise. We reached the speed and the buzzing noise came. The mechanic went back and told his manager that there was a humming noise. They couldn't tell me what it was, but they guessed it was the rear wheel bearing and stabilizer. Thank goodness their guess was right.
You need to call the repairman to come with you. Have it repaired under warranty.
I sat in several MG4s, and they kept telling me that the car didn't make any noise. Fortunately, I didn't let them convince me. Now there really is no rattling or buzzing, and it doesn't make any noise at all.