Followup to previous report:
I took my '98 Sentra to my friend's cheap mechanic who does well with old American cars. He said the noise was "normal for Nissans that age" and not to worry about it. It also had low power on acceleration.
I took it to a friend at TechShop who used to be a professional mechanic, and he changed out the timing chain guide. The plastic layer had worn down to the metal, causing the clicking noise. However, I had followed up on the noise before the friction damaged the timing chain itself. The parts were $50 and he charged less than $200 total to fix what the pros said was a $1200-2000 problem.
Car runs great again.
I don't know if my friend's mechanic is just ignorant about this issue and the cheap fix, or if he's hoping he can get to charge for the full timing chain replacement or a new engine in a few months. Telling me to ignore the noise is negligent when it indicates damage to the timing chain.