Oral-B Vitality Type 3709 only works for few minutes

Hi. I have this weird issue with new old stock of this 3709 model where it would work for few minutes or maybe longer without brush head attached. When it nearly starts to act up, the toothbrush is behaving like the battery is nearly empty where the metal end vibrates very weak until it stops. When it stops, I could hear some sort of humming sound. I tried to move the metal end left and right, it seems like the motor inside is struggling to work. It feels like it's trying to move to vibrate, but it failed.

I let the toothbrush off for few minutes and after that, it starts working again at full speed like fully charged battery until few minutes of using it that the metal end gradually gets vibrate weaker and eventually stopped working again. Note that I didn't put it in the charger. I let it cool down for a while and the cycle repeats. Hopefully someone can help me. Thank you.