Outside exhaust fumes entering my 2001 cabin

Hello everyone, first post to the forum. Hoping someone here has encountered the following problem and has the solution. I have a 2001 tacoma extracab sr5 with V6 engine. Lately there has been a "clunking" sound coming from my dashboard every time I accelerate and when I come to a sudden stop. It sounds like some sort of flap opening and closing. I've noticed that the smell of exhaust fumes is pretty strong while I drive, especially if I'm behind a diesel truck. I've done a little research to find that my model doesn't have a cabin filter. Anyone know if indeed it is a faulty cabin air flap of some sort and how to go about repairing it? Thanks in advance.