MMMMMMMM can be used when you are in a very stressful/uncomfortable situation by a specific entity. Usually in this scenario its followed by something else.
Example one: *getting very creeped out by an enemy in a video game* MMMMMMMMM I DO NOT LIKE YOU

Example two: MMMMMMM THIS HERE BE SOME MAJOR BULLSHIT
by Mr chaotic neutral July 21, 2019
Me: okay im gonna make a corn dog
Microwave: mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
1 minutes later
Microwave: mmmmmmmmm *exploded* KJWEUR *exploded* KJWEUR *exploded* KJWEUR

Me: ...