vol

The chain was rubbing the front derailleur, so I went to the LBS and the guy fixed it. But upon returning home, I found that when the front derailleur is on the middle gear (chain on middle front cog), if I pedal backward or walk the bike backward, the chain has a constant clicking noise. No such problem when on any other cog or when forward pedaling. Possible cause?

FBinNY 

If it's happening with the chain at either end of the cassette especially the outer end, it's probably.

Chainring teeth are cut with points to pick up the chain coming from a variety of angles. But cassette teeth have hard square profiles to improve shift response when moving to larger sprockets. This tooth profile is unforgiving of chains coming from an angle, but it's OK beacus ethe chain is always fed straight on by the derailleur. When back pedaling there's no guide, so if the chain comes from the side it can snag before engaging, and may also autoshift to a more inline sprocket.

There's another issue, if using an inner chainring with the outer cassette sprockets, in that the chain may brush the outer ring as it comes by. The clicking you hear may be a pickup pin touching the chain.

vol

It doesn't seem to have anything to do with the cassette, because the click (which is not smooth sounding as supposed to be) happens when the chain is on any/each of the chain ring (middle and end), as long as it's on the middle cog in the front. The noise is also from the front.

FBinNY 

You've got the bike, not me. Get low, and observe carefully as you backpedal and listen for the click. There's always the chance that it's something stupid like a crank arm touching the loose end of the FD cable.

vol

I found the culprit. I doubt any of you would have guessed it: it's a tiny (less than 2mmx1mm) residue of manufacturer's wrapping material that didn't get peeled off from the top of the front derailleur cage , which is rubbed by the chain when the chain is on the middle cog. Only when back pedaling because of the way the little piece is stuck on one side of the cage. It's very hard to see. I put laid the bike down and looked hard to find this little red colored "foreign object". See attached picture (with a regular-sized paper clip that I used to access this piece of plastic wrapper). From the partial picture and words on it it seems to be a label/instruction about how to assemble/install the front derailleur.

"Something stupid", indeed (more stupid than crank arm touching cable ends, which I know from past experience).

Add this to possible causes of chain noise on a newly bought bike.

FBinNY 

There's nothing like having your own eyes and hands on a problem. That's why I told you to take the time to OBSERVE for yourself, rather than play guessing games via the internet.