I've got three extended batteries and covers. I do have a little bit of creak in each, but it does not bother me. However, remember, the antenna is in the battery cover. I would caution on putting tape or other material inside the rim/back. Make sure you are very careful not to interfere with the numerous antenna connections and don't put too much that would pull the battery cover away from making good antenna connections with the phone. If you start having signal issues, look to the padding first.
I might suggest putting a little silicone lubricant on the edges using a Q-Tip. Don't know if that will work, but might be worth a try.
I asked the same question last week, I thought I was being a bit paranoid with the creaky sound and was happy to know that I wasn't alone. I hope an extended case will stop this. I don't hold my phone much due to using a headset but It is still annoying for the times when I do. I use full time since the update S***** up my stock battery. When held it creaks on the side by volume rocker. But don't appear to be loose. Besides that annoying sound The battery Rocks! Curious...Will extended cases stop the creak?