I find some interjections to be too tricky to pronounce.
Can anyone help me with this one? Is there any specialized dictionary that reports the pronunciation of interjections?
I'm not sure brrm is an "interjection", but let that pass.
It's an example of onomatopoeia. That link gives many more onomatopoeic words, but curiously it doesn't have brrm, which is pronounced broom.
Many if not most people say/write vrrrm for the same sound, and in both cases the word is often repeated. The precise number of r's may vary, and sometimes it'll be written as, for example, vroom vroom (it's often intending to convey the sound of a racing car revving up).