Originally Posted by Elizabeth
Some of the low frequency rumble is the fact it is a hatch. All hatches are more susceptible to make a deep rumbling sound due to the shape of the interior.
The fact your car is missing that hatch bit which goes from seat to rear covering the cargo space probably makes things worse.
that piece really IS a part of the car. and no way should it be missing.
You are probably right on this, the OP needs to get the hatch cover. With the muffler, rear wheel wells, and exhaust exit underneath or in the hatch area, I can see some low frequency sound making it to the front without the cover in place.