Could your chain be too long? If so, when in the small chainring the smaller cassette cogs would be making the derailleur pulleys try to wrap up the excess chain which a short cage RD might not be able to do thanks to an extra link or two in the chain. Does the noise in the small cogs persist when you are in the big chainring?

Another chain related possibility is that you routed the chain through the pulleys improperly. You would most likely get a God awful clattering noise if the chain is riding outside and on top of that little metal tab on the derailleur pulley cage. You and your LBS mechanic could not possibly have overlooked that, right?