I've been chasing a "thunk" noise around for a while on my STi. Frequently, when I hit a hard bump in the back, you feel the bump through the car and then a split second later there's a "thwunk" noise, almost like you were dropping an object on to something hollow. It's not that loud, but it's sort of irritating.
I traced it down to my Rally Armor mudflaps! If you pull the outside edge of the flap forwards and then let go, it makes a similar noise and if you kick it at just the right angle with your head inside the hatch... it's the sound I've been looking for exactly.
I think this is the surface of the mudflap hitting the fender liner and it's echoing through the back of the car. The rear flaps are attached high on the outer edge and then in the middle on the inner edge so there's a fair amount of surface area that isn't fixed in place by any of their mounts, so it can hit the fender liner when it moves.
Now... the question is. Is there any way to silence this noise that doesn't involve getting rid of the mudflaps? I like the look and with all the snow and sand on the roads around here I'm sure everyone behind me appreciates their functionality. Ideas??