I remember reading on the forum awhile back about GT-Schwinn-Mongooses having a creaks occationally. It seems the fix was adding lock-tite and grease, but I do not remember if it was the crank or pivots. It's my brother in-law bike, and i'm helping him tune up his bike this weekend. So haven't heard the creak myself, and even if I did hear it, I am not sure I would be able distingish where it came from :D .
If anyone has info on the fix, I would appreciate.
Bunk

P.S. I've searched the archives of the forum, but couldn't find the info.

I had a creak on mine that drove me crazy. Turned out to be the seatpost. The seatpost collar was turned the wrong way to tighten the seatpost/seatpost tube cutout efficiently. Cleaned, lubed all parts, and reassembled, creak gone.