I hear a squeek/grind coming from our 2004 Trek T-2000 that we've narrowed down to coming from the front BB area. It developed during a rain ride. Well, I guess it could still be a loose chainring on the captain's cranks, or timing chain ring on the rear cranks, or a loose crank arm of the front set. But I guess I'll start with the BB bearings and eccentric.

I plan to try disassembling the cranks and eccentric to clean and regrease. I found this: https://www.tandembicycling.com/439/t...bottom-bracket, which looks helpful.

However, did I hear somewhere that some Teflon tape should be used when reassembling a BB or an eccentric? Not sure.

Lots of ideas back here. On our bike there was a little paint in the BB housing. This caused a squeak at 12000 miles.
I know a guy who's similar squeak turned out to be a broken frame (carbon fiber-single bike), but I really doubt that's your problem!
I've never used teflon tape. I prefer a LIGHT coating of anti-seize, but a light coating of grease is easier to work with.
Bushnell has an article on the list of things to grease to eliminate noises.

FSA recommends Teflon tape on the bottom bearing brackets, the ones that screw into the bottom bracket.

Have never used teflon tape for BBs.
However, go with manufacturer's instructions.
Squeaks are darn hard to isolate.