Could have been a worn chain or also could have been a Shimano chain inside out. I've done that before, among the 43,231 other mistakes, but who's counting?

qcpmsame-

Boulder Bicycles has NOS Regina chains. Be prepared for sticker shock.

Just ran into a similar issue, the only noise I heard was on the small ring and it seemed to only occur under pressure of pedaling and not in the stand. What I discovered (entirely by accident, by the way) was that one of the link pins was seated too far inboard. In other words, the pin was very slightly too long on one side of the chain. I thought to myself, "Naw, this couldn't possibly be the cause" but I torqued it slightly with my chain tool and heard a "pop" as it seated into place. The chain is smooth and quiet now. I've never experienced that before.