MC’s get so quiet you can hear
A motherfucking dog whistle when I walk by

from Eminem – Untitled Lyrics on Rap Genius

Meaning

Dog whistles are deafening to dogs, but too high-pitched to be heard by humans.
When Em walks in the room, MC’s get so quiet (as in they wouldn’t dare spit against Em) that you would be able to hear a dog whistle.

We already know Eminem is a dog-lover, anyway..

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