If you have a bike with a rear deraileur, or a singlespeed with a chain tensioner (which basically looks a lot like a rear derailleur, or if you check this post, you realize it can be EXACTLY a rear derailleur) and one day you realize there's a clicking noise coming from the rear wheel when you pedal - so you start playing with chainline, move the top pulley left and right, and just can't get completely rid of the clicking... did you perhaps remove the wheel (for maintenance, like replacing the tire, tube or cassette or truing the wheel) before the ride? It may be that the chainline is just fine - the way you adjusted it originally - but the axle of the rear wheel is not perfectly perpendicular to the midplane of the bicycle. Loosen the rear wheel bolts or QR and make sure you pull the rear wheel backwards while you're tightening them back.