Sound damping: Rear cabin vent

I've done quite a bit of research on sound dampening the cabin to decrease the ambient noise while driving. I've even spent time on other forms for Jeeps, Corvettes, and other trucks. I haven't seen anyone address this particular issue.

I used a sound level meter to test various locations around the cabin to determine the main sources of sound ingress (I'll post the actual numbers in a full write up later). Engine noise is the biggest contributor to noise. Other than that, the highest readings came from the back wall. The cabin was on average around 77 db at 70 mph, while the rear wall was 81 db and even spiking at 83 db. This is a 2000 single cab with a 2.7L.

Yesterday I applied Frost King to the rear wall. While removing the the seat belt tensioner and associated trim piece, I realized there is a cabin air vent behind the tensioner that goes up and out the rear of the cab through the black plastic piece visible from between the cab and the bed.

Doing a quick reading on the sound level this morning, there is definitely an increase of noise when the sound meter is pointed at this vent vs just the rear wall. So I'm posting here for ideas on adding some type of sound dampening to the vent, if possible.

FYI- this vent is used to equalize sudden changes in air pressure, like when closing the door. Also for allowing fresh air to flow through the cabin.

The main problem is the louvers are integrated with the sheet metal, so getting access to the cavity is impossible. My first thought is to spray rubberized coating through the holes and just coat as much as I can and hope for the best. However, I can't get in there to clean the metal to ensure the best adhesion of the rubberized coating to the metal.

Also, is there a way to remove the exterior vent piece? To replace with new would cost up to $150!

Any other ideas?
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To orient yourself, the vents are at the red arrows, which are normally behind the seat belt tensioner and its cover.


A close up of the vent, with some visibility of the interior of the cavity.


And here's where the outside vent pieces are located.

 

What if you pushed a bunch of pieces of acoustic foam through the interior-side slats?

 

another idea would be to hang/glue/screw a piece of rubber sheet over the interior slats

 

Bingo, that's the idea I arrived at after this post.
I bought a heavy rubber tread pad from Harbor Freight. I took more readings with and without this piece covering over the vent, just with taping it up.
-83 db without the rubber, plastic panel or seat belt tensioner in place. This is by far the loudest noise within the cab.
-79/80 db with the rubber in place. It doesn't seem must like an improvement, but because the logarithmic scale of decibels, that about 4 times quieter.

I plan to spray rubberized coating on the inside of the cavity and then hang this rubber pad on the inside. It won't be easy, I will have to cut some of the metal to make a slot big enough to slid the pad in.

 

If you could fill the cavity with fiberglass insulation, rock wool, acoustic foam, or poly fill that would significantly cut down the outside noise from getting in without sealing the interior cabin from outside (prevent you from breaking windows). Your rubber pad must just be reflecting the noise back into the vent which is good as well as long as it doesn't seal it. The spray rubber coating only adds mass to the sheet metal which changes the frequency at which it will resonate.

 

Update-
I kept putting off this project because it seemed like a huge pain to do. So in the end I didn't add the rubber mat, partly because I originally didn't account for the bolts of the seat belt tensioner sticking into the cavity thereby preventing the rubber mat from sitting flush against the opening.

Instead, I relied on meticulously applying pieces of Frost King duct insulation to the inside if the pillar panel and tensioner cover. The idea being to add mass to the panels and maybe reflect the sound coming in.

I took sound readings again and it made no difference. Still getting between 82-84 db.

I wish I could get to the outside vent assembly without having to take the bed off. I would have more options if I could get that piece out by modifying it and having better access to the cavity.

 

Nice work there. I have sound deadened a couple of other cars over the years. A side benefit came in the summer when the AC didn't need to work as hard from the added insulation.