From another post:

"The only bummer on my entire car so far has been the creaking and groaning plastic around the seatbelt connection, under the rear windows. Today I found out that it is not even those pieces that is making the noise! its the plastic that runs along the top of my rear firewall bulkhead. The spot where it runs under those seatbelt caps is where it is rubbing against them, and making the creak.

Solution: I took a credit card, and slid it under the cap, and gently pried it upward so there was a miniscule gap between the cap and the wall top plastic. VOILA! Silence. The back bulkhead wall can now gently flex under hard acceleration, and make no noise.

If anyone else is having this problem, I wanted to post this so you can regain your sanity."