A/c is working perfect, blowing ice cold from proper places.
only way to describe this....every 15-20 mins. there is this "thwoomp" noise...hard to identify the source because it seems to come from everywhere at once, like surround sound.
I know....if this is my biggest problem with the Ram...I'm doing O.K. but I'm very curious to know if anyone else has this and/or what it is...

 

Does the noise happen to coincide with decreasing engine vacuum, as when accelerating or pulling a grade? If so, you're hearing the vent doors flapping. You can test for this easily enough: crank up the AC on a safe stretch of road and mash the accelerator. If it thumps and the air starts coming out of the defroster vents you've confirmed that this is what's happening.

Though the itty bitty check valve sitting over the hot engine is a regular failure point, this can happen just because old dry vacuum lines leak a bit and there's not really all that much reservoir capacity to make up for it. You can choose to live with it, or replace all of those vacuum lines. It may still happen on really long grades, and adding more reservoir capacity can help to lengthen your hold-up times.

 

Is there any way to fix that problem with the vent doors from closing when I accelerate? I have this happen all the time and it really sucks when my truck is hot and the a/c cuts off.

 

Get the plenum plate and install it. Then replace all your vacuum lines. That is the only way to fix it.

 

Although a blown plenum gasket can in fact contribute to this problem, the problem can occur in the absence of a blown gasket, and can be absent in the presence of one. This problem exists in vehicles without plenum style intake manifolds, too -- I had one. It could be a failed check valve (a common failure, eight bucks for the part at the dealer), a disconnected or broken line upstream of the valve, or just the common case of old dry hoses being porous enough to leak down.

 

+1 on replacing check valves and inspect for vac leaks before doing something major.