Exhaust Vapors in Cabin

The problem happens when I'm driving with the windows up. I've been noticing this happening for about a week already. At first I didn't pay much attention to this because I was usually driving in traffic and behind an older car which I presumed was on the higher end of exhaust gases emitted.

When I started paying attention to this was when I was driving for a good while behind no one and I was still smelling exhaust gases in the cabin. To test, I rolled the windows down to air out my car they were down for a good 10 minutes before rolling them back up. Still not behind anyone, a few minutes later, I was starting to smell exhaust vapors in the cabin again. After completing the first test, I retested but the second retest I did with the air recirculating to prevent outside air from getting in. After driving for a while, I did not smell any exhaust vapors coming into the cabin of my car.

When I drive, I drive with the AC off, fresh air coming in, and windows rolled up.

I just did a 1hr drive and the gases were so bad that I now have a headache. I did roll the windows down intermittently, but because of rain I couldn't properlyr air out my car during this drive.

Where should I start looking to find this exhaust vapors issue and fix it?

hmm, sounds like rear...
trunk smells worse?

engine COLD!
jack it up, with a spray bottle loaded with soapy water.
start it
slide under (right away!)
listen for leaks, spritz any suspicious areas

bubbly water = leaks

I'll do that. But I'm thinking there's also a hole somewhere in my car's body that will allow for these gases to come into the cabin?

oh, and it makes it easier if you can jam a rag in the tail pipe, sometimes they pop out though, cause you're trying not to plug it. Resist it. Vicegrips help.

leak inside, hahaha, really. You've got, what, like a thousand? none are prone to breaking themselve, so look where you added stuff, bu basically look at them all (edit: rubber seals)

Not sure I want to actually jam a rag down my exhaust pipes. That's just asking for trouble.....

you dont exactly jam it in... you just plug it in enough where it wont fly out from exhaust pressure. you could easily pull it out.

Its not like you're sticking banana's in there lol

So no exhaust smell when you have it on recirculating air? but the outside setting you smell it? Inlet is at the cowl under the wipers, so possibly an exhaust leak up near the engine somewhere that is getting sucked in at the cowl from under the hood? Do you hear any exhaust leaks anywhere? Completely check your exhaust system for leaks around all connections and look for cracks in the manifolds or flex pipe. It is possible for an exhaust leak to be pretty quiet. You can also try to plug the exhaust with a rag as suggested before and hold your hand about an inch away from each connecting point from the manifolds back, to feel if any fumes are escaping.

Do you have any CELs on? Could be something like a leaky EGR/PCV hose or soemthing similar.

My only CEL is for a misfire that came up this morning on my way to work. I don't think it's related to this.