Is it perhaps standing on uneven ground?

It must first be clarified whether the humming is coming from the heat exchanger located on the balcony. This sometimes hums loudly and sometimes quietly. If the humming is coming from the air conditioner in the room, then touch the housing with your hand to see if you can feel any imbalance. If so, it may be coming from the built-in fan. This noise occurs at low speed. Speed 1-10%. Can be adjusted continuously from 1-100% with the mobile. In my case, the imbalance is present at low speed. I will take further action. It should also be noted that the air conditioner stands firmly on the floor. It happens that the floor is uneven. I have bought an anti-vibration mat for the washing machine and also vibration dampers for the washing machine and used these accordingly. The heat exchanger on the balcony has also been fitted with vibration dampers.

My midea always hums a bit. I think that's normal as there's a compressor working alongside the fan. If the noise isn't or doesn't get louder than before, you don't need to worry. At least I don't think so.