Speeding up or slowing down a clip introduces metallic noise when adjusting for pitch
Most of the time you should not rely on the automatic Maintain Audio Pitch setting because it causes the audio to be distorted (both when speeding up and slowing down). 

Instead, leave that checkbox unchecked, and add one of these filters instead:


Pitch Shifter (Obsolete)
AUPitch


And play around with the settings until it sounds better.

See SPEED UP Video with No Audio DISTORTION in Premiere Pro for more info.