Creaking noise per pedal cycle, turned out to be part of the saddle rubbing on the saddle rail. Solved it with a qtip and some grease.

Creaking noise per pedal cycle, turned out to be loose chainring bolts, my chainring was actually about to fall off! Tightened 'em up.

A year or two later, started getting a similar noise, I check my chainring bolts, some are maybe not quite as tight as they could be, I tighten, still noise, tighten, still noise, and end up shearing off the heads on two of them. Replaced them with steel chainring bolts, a little grease on the threads (not the shoulders), cleaned-off chainrings and spider, bottom-bracket (UN-55) pulled and cleaned, bb shell cleaned, reinstalled with a spacer, finally it's quiet again. Not sure exactly what the cause was.

Adjusting front derailleur, can't figure out why I'm getting chain-rubbing noise, because I can see clearly there is clearance all around the chain -- turns out the FD is too low and chainring teeth are just barely scraping it as they go by, sounded exactly like a rubbing chain.

One time my plastic ziptie-on chainstay protector was rubbing on my tire.

Another time my rear blinkie was rubbing my rear tire, because my tires are so fat. I had to hacksaw off half the mounting tab off my rack, and install the blinkie hardware a little higher with just one bolt.

Just recently, I heard&felt an irregular clunkiness in my drivetrain, I logicked while riding that I must have a bad chainlink, and I was feeling it go through one of the derailleurs, or go onto or come off of a cog or chainring. I was smart enough to talk it up the killer driveway instead of leaning on the chain. When I got home and inspected, sure enough one side-plate was cracked in half.