I installed a 3/8 floating dining room floor of engineered hardwood using TOH website's instructions. It looks drop-dead gorgeous. I leveled a low area, but I guess not enough. The last 18 inches near the entrance feels spongy when walked upon and the boards squeak when walked upon. I'm assuming that I should shim this area. I'm guessing 1/16 to 3/32.

What I would like to know from you is what material I should use as shim and should I put the shim between the floating floor and plastic under-layment . Or should I put the shim between the under-layment and the sub floor? Thank you