IMovie - iMac (intel Core 2 Duo)

Strange happenings - any ideas how to overcome?
1) Select a clip.
2) Trim clip by splitting at play head. Clear trimming not required.
3) Add another clip
4) Add transition between clips (cross dissolve)
5) On play back, during the transition, I can hear sound that actually
relates to that part of the clip that I have cleared.

Strange but true. The video source is DV downloaded from a Sony.

Thanks
Colin