I had pretty much the same symptoms that gradually got worse, and it turned out to be a bit of dirt in the plastic cup on the cartridge bottom bracket. I also had a similar symptom on my road bike quite a while ago, and it turned out to be a CO2 cartridge in my seat bag. If it's not making any more ominous sounds than a tick, and you don't start feeling it in your pedals, it's fine to take the time to locate it before you start taking things apart. Or, as someone suggested above, the shotgun approach can work.