I think those sounds originate from the marketplace, each one signifies resource and/or luxury transaction being made by the AI.

I think that if you switch view to marketplace quickly enough, you can see trades being made.

Still annoying though.

Since AI seems to be making transactions always at the beginning of turn, wouldn't it be better idea to decouple them from the UI completely? Just do them in background and present the overall result. They also seem to slow down turn completion, since until the trades are done my commands will not register.

If this post seems a bit ranty, its because this weird "asynchronous" turn system produces quite an amount bizarre situations among all endless games.