[rodster]

You asked for proven methods and this is not that but it may offer an idea ...

I had the center dash trim and the rear vent trim off last week. The back of each vent is a male tube, the end/circumference of which is wrapped in foam with a tape backing. One of my vent's tape was malformed so the male end wasn't fully insulated from the female tube it went in. (Hope that makes sense.) Wonder if the area where the two pieces join could be the source of the popping?

Not a very good picture of the end of the female section, but this is the left center vent tube. You can see the flare on the end where the insulated male end goes.