Exhaust noise - cabin vent

I finally got my old money pit back on the road (1994 SW20 - now with a Celica GT4 3S-GTE engine swap of unknown origin or vintage - I am sure I'll be back with plenty more issues), and I am not used to the exhaust drone any more (been years off the road).
 
The little cabin vent behind the driver right shoulder looks to be where the peak of the noise is coming in (my 3" exhaust exist on that same right hand side). Is it possible to dampen that little vent or block it up without suffocating? I'm assuming there's a ton of air leakage elsewhere anyway. I couldn't see a way to remove it from the outside, and on the inside it's just some holes in the internal b-pillage moulding, not a piece to remove.

Yeah I guess I'm just assuming. I'll pop a temporary makeshift muffle patch over it and see how I go.

Is that something I can find myself, or does the engine need to come out ? (Please god no)

You have a hidden camera set up near my car to record the hilarious nature of this test, don't you? But cheers for the tip

Done the funnel in the hose trick, works brilliantly for finding hissing leaks around an engine and no reason why
it won't work in this case. Quite precise.
I am in the middle of soundproofing my Sdub and behind the panel behind the driver's shoulder, is a hole for the seat belt mounting and a hole for the rear crap speaker (removed). That cavity brings a ton of fan and rear tyre noise into the cabin. The same cavity on the other side also, but no fan. If there is one place to fix, that is it. I have lined the cavity with stick on vinyl and filled the cavity (mind a couple of cables) with high density foam. All up 2hrs and $40 (at a guess.) I need to complete some other tasks before driving to confirm the result. 

Sadly I've been too busy to have a crack at it yet. And/or when I do have spare time it's way too bloody hot to be mucking about with the car. I have a few weeks off work now though so I should be able to get in there in a few days or so.

Check the exhaust over for any leaks.  Second option - electric exhaust valve motor.  They are throttle body like valves, that can be installed towards the end of your exhaust system and hooked into the vehicle wiring (depending on ecu or aftermarket switch maybe required).  At low throttle the valve sits partially open, reducing airflow and sound, at open throttle they open for full flow.  ATS Racing facebook page had a post a while back.

More on the engine drone. I was sitting at the lights the other day and a big thumper pulled up next to me.
Either a Harley or a Trumpy or something. With my windows closed I could hear and feel the bike idling and it
was all coming through the floor. So although I have put some vinyl sheets on the floor, it is not enough. Thinking about pouring fiberglass resin on the floors to increase the weight....might try one side first. Bonus would be that my subw would cause less doof, doof for my fellow roadusers. There will undoubtedly still some engine drone in the cockpit but any reduction makes the music nicer.