I believe it's "Meep meep".
Which, in translation, means "Nyah nyah nyah nyah nyah".
I took audio from the Road Runner cartoon "Zoom at the Top" and slowed it down to 1/4 of its normal speed. Surprisingly, when listening to Audio at 25%, the RR says "Mee mee" one time and "Mee bee" another time. That is, there is no ending "p" sound.
Meep meep :-D
The Roadrunner use to say "stop ya ****** bastard" but due to censorship it was changed to beep-beep
Mine said beep-beep,which translated into take me to the gas station and feed me