I'd say it sounds like "nap... nap nap... " when the body of the car is bending after the patches of the road. Difficult to describe but I would not compare it to wooden floor creaking.
When I went back to my dealer they torqued the bolts of the bracings behind the back seats but that did not help
I actually made the same but put my wife on the drivers seat to drove around and tried to find the noise by myself in the back seat put still cannot tell where is it exactly coming from. Once in a while it sounds coming from the roof but when you turn your head again it feels like coming from...
First of all greetings from cold Finland and sorry for my bad English (hi)
I'm an owner of year -06 Mazdaspeed 6 (or here its named Mazda 6 MPS). Recently it started to hear quite loud squeak some where from the back side of the cabin. It's really hard to localize exactly where. But it's some...