strange rattling in 96 LSi

[bambambam]

hmmm...wondering if you guys have run into this...
96 Metro LSi 1.3L, auto
getting an odd "chattering" noise when the car is about to shift from first to second, and sometimes from second to third. It only does it for a second or two. At first I though the alternator belt was coming apart and slapping the firewall, but thats not the case.
Anybody else had this problem? Its been doing it for a few weeks and doesn't seem to be getting worse, but it certainly is bugging me!
Any help would be great, thanks!