Question about Shi, Zhi, Chi

Is there a regional difference in the vowel pronounciation of Shi, Zhi, Chi?  I don't mean the Taiwan pronounciation (under which they would be si, zi, ci, and thus have a different vowel pronounciation), or from erhua in some words.

But, sometimes (most of the time with Cpod) it sounds to me as if the vowel, while obviously very r-like, has a bit of an i type sound in it.  Whereas, sometimes (particularly when I know the speaker is northern) it sounds firmly r, with no hint of an i type sound.

Sadly, I couldn't figure out any less clumsy way to state this...