That is a bad sign. If the buzzing starts when you press the start button and stops when you release its almost safe to say that the motor has a dead short or grounded out and causing that to happen. You can check the start switch by getting a multimeter and with the dryer unplugged take the wires off the switch, place a lead on each pin and press the start with meter set for continuity. If it shows continuity when presed and stops when released your motor is shot and needs to be replaced.
Now if you press the start button and the machine starts to hum and continues to hum its possible that something has gotten caught in the blower wheel and binding the motor up. Depending on the make and model the location and access to it vaaries.
If you can turn the drum and then the dryer starts you have a bad motor. The way the dryer works is when you initially push the start button, electricity is sent to the start winding, or stator, of the motor. This will in turn start the motor turning. From there the stator will drop out of the electrical process and at the same time the run winding, or rotor, will take over and the motor will continie to run. Your stator is damaged, has a burn spot, etc so by you turning the drum, the belt around the drum is moved which is attached to the motor pulley. You basically take the place of the stator. Long story short, you have a bad motor.
If you need further help, I’m available over the phone at https://www.6ya.com/expert/jason_4570d45317834dd3