Really loud buzzing from a brand new Milwaukee weed eater

On the head/feeder be sure the red lines are aligned to the correct location

I caused something similar by not fully locking the shaft into position at some point on a trimmer I have that supports different attachments.

Sounds and looks like something wrong with the gearbox in the head. Or the shaft going into the string attachment itself. That should spin up to full speed in like 1 second. Take it back.

Sounds like it’s grinding gears. Return it. It should be out of the box ready, and that is definitely not.