Trouble shooting "sounds" falls under the heading "Black Arts". Short of contracting a Druid Priest, I would get the rear wheel off the ground, and simply spin the wheel and search for the source. I would be checking my chain for "tight spots". Seeing a reddish dust on the chain may be another indicator the chain may be seeing the end of it's useful life. Chain guards can rub, tires can rub. A constant clicking could be anything from a bad bearing to a lodged rock. Or a chain, singing it's swan song. That 270 might be going "clickity clack". If that is the case, it's one of the tire's better attributes.
“many a trip continues long after movement in time and space have ceased”- Steinbeck, [I]"Travels with Charlie"
[FONT="Century Gothic"][I]Sometimes your only available transportation is a leap of faith[/I] [/FONT]