Rattle or vibration when closing door

I have a 2000 Silverado 1500 regular cab and long bed. Within the last month, I have noticed a distinct vibration sound coming from the cab when I shut the door on either side (windows up). I noticed a square like object on the outside of the back of the cab on either side and wondered if they were some type of air pressure release vent. You can see the back of the cab vibrate when the door is closed. I can not find anything that speaks about what these vents or objects are. I wonder if they are vents, that some how they may have become clogged or blocked. You cannot access them from the outside because there is not enough space between the bed and the cab. So it they are part of the problem, I guess I would have to access them from inside the cab by removing the back panel of the cab above where the jack compartment is. I do not want to attempt this without knowing if I am correct in presuming their involvement in the problem. Can anyone help by addressing the problem and hopefully a solution and where can I find a diagram and instruction on how to access these vents or whatever they are called.

Thanks,

Luggage

I have never heard of these. But I would guess to same as you did. The trucks now are so air tight they would have to vent some air, do they vibrate with the windows cracked. I will get a hold of my brother he's an autobody guy see what he says then I will get back to you.

it is a vent.

I just got back the boards and see that TweakedGM says the cab vibration is in fact the vent. Do you or anyone know how to accees the vent to resolve the problem. The clearance from the outside is next to impossible givent the space between the bed and the cab. I do not want to go trying to opening the panel behind the seat without some guidance.

Thanks

luggage

Doubt those are the problem but bet they are vents. There are also pressure vents inside the doors as well. If you still insist on access these vents (on back of cab), you will have to remove trim pillars in rear. I think I know which vents you are reffering to, (think the are black plastic, kinda rectangular but for most part square) but can't think of the exact location on 2000 models. But if the are kinda close to corners of cab, you might be able to access them after removing the pilar trim panels. If not, you'll have to remove the carpet on back wall of cab and any other trim surrounding it.
Still trying to think of what this rattle you speak of could sound like. If you had ext cab I'd say are the rear windows locked in place but you mentioned yours is a reg cab. Do you have a "beer window"? Is that latched properly if so?