Your snake cannot vocalize, but they can emit air... and air can squeek as noted.

I have a few that are ALWAYS going into a low hiss when I reach for them. Often that comes out like a squeek if I pick them up in mid hiss - I know... I am a slow learner.

Hold the snake to your ear... systematic whistling could mean an RI, but as Ed noted, if you can rule that out, then there may be other things at play.

bruce