So I decided to turn my old Gianni Motta road bike into a fixie. I have a new Alien Bikes wheelset, so I stripped off all the junk and put the 42 tooth chainring on the inside, and it seemed like perfectly straight chainline to the 16 tooth cog. I got the chain pretty tight, but every time I ride it, there's a constant clicking. I feel it pretty much every time the chain passes a tooth on the rear cog. I went to a local bike shop, and the guy said that maybe I needed a Single-Speed chain. I looked at it, and it looks almost exactly like the one I already have on it. Any other ideas on how to fix this?? Thanks

mark the noisy chain link and ensure it's the chain link. Spend $10 on a new chain. It's clearly the chain if it is consistently clicking on that link, and even though the new chain looks the same or whatever, it's not a pricey buy and will cure your woes (if it is the chain).
Just makes sense that you need a new chain.