Your problem description doesn't ring a bell for me. Here's a URL you might find useful - it contains WAV file samples of some common voice distortion problems and matches them to a category and some suggested methods of resolution. If you can find a WAV sample here that sounds like your distortion pattern, you might find the solution as well.
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_white_paper09186a00801545e4.shtml
If you can't find a matching sample here, it's always good advice to check your router QoS config (make sure you are properly prioritizing voice traffic, no excessive jitter, etc). Also use the builtin statistics tool on the phone to look for problems with the voice traffic flow by touching the Information button (the question mark in the center of the Messages/Services/Directory/Settings quad) twice in quick succession while on a call.
It's also probably a good idea to try the new phone software loads available on CCO. The new 5.0(2) signed phone loads have been working well for us internally and for all our customers (5.0(1) had some issues). Try it out on a couple phones before deploying it widely, because you will not be able to downgrade from a signed load to an unsigned load.
