I don't know if I'm hearing the same thing you're hearing but...
... Lower the rear backrest, see if you hear it more.
The dull slapping that seems to match the exhaust pulses may be coming from the rear body vents hidden behind the rear bumper in the corners of the trunk. At idle, exhaust resonnance can make the flaps oscillate in time with the pulses. I know my stock 86 with stock exhaust does it when I park it home backed to a brick wall. Your car with the obviously loud exhaust may be more severe.