Most of the creaks come from chassis flex. A strut brace would stiffen up that area, but you would lose trunk space. Short of that, try stuffing the area below it with towels.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>