[Dynamo]

your problem is real simple.
the magpul MIAD grips have a design flaw that magpul overlooked.
befor i get into further detail, check to see it your motor is still alive.
remover your motor from the grip and reconnect the wires, then pull the trigger and see if it still runs. if it doesnt run, well you are going to need a new motor.

ok if your motor does still run then good, i can now explain to you what is wrong with the MAID grip and how to fix it.
the problem with the MAID grip is its end plate motor hight adjustment screw housing(the brass part molded into the end plate).
the problem here is that magpul screwed up and didn't design the screw housing with the needed recess for the endbell of the motor(the bottem of the motor that holds the bearing for the motor shaft). this causes the motor to sit on the endplate and not on the hight adjustment screw, which makes the motor sit far to high(causes the "REEEEERN" sound you described).

now for the fix.
you are going to need some tools to remove material from the inside of the screw housing, a dremel or even a drill.
basically you are going to make that recess that magpul forgot to make.
you will need to remove enough material(about 2mm), and expand the diameter to 10mm. after that make sure that the endbell fits inside the newly created recess, and then reassemble the grip with the motor inside.
now you will be able to adjust the motor hight adjustment screw properly.
*note: the hight adjustment screw does not have any means of staying in the position you set it to, due to the motor vibrations it will unscrew and lossen, lowering your motor. you will need to use some loctite thread locker to keep this from happening, after that you should be good to go.

hope this helps.