I have a 1999 Avalon and found several years ago that Toyota has what they call a " Technical Bulletin" out about the suspension system problems inherent in this model. Unfortunately, I found out a few hundred miles after the warranty was out and neither Toyota nor the dealer Cavender Toyota helped me. In fact they told me a few times they could hear anything. The car is very noisy expecially in winter or when the car is cold. I have a friend who has a 1998 Avalon and she has the same problems. She put new struts on the rear recently. I recently replaces all four struts and three engine mounts that were worn/broken. While the car feels better and rides quieter it still has noises I haven't been able to isolate after several years of trying and having mechanics try as well. Sorry I can't help identify your noise but I had to tell you you are not alone. I will not buy another Avalon and maybe not another Toyota. Mine was made in Kentucky.