mine used to do that too. also when i would go up a hill or when ever i was drawing hp from the engine like that for a long time, the vacuum would drop, and my vents would close up, but after i was over the hill, the vents would open back up, i assume cuz there was more vacuum to force them open. this problem went on for years and it turned out that there was a leak under the hood in the vacuum lines. i fixed that and it doesnt do that anymore, no more pops or loud noises when the vents change. check to see ifyou have any leaks in your lines.
1999-2007: ___1988 Bonneville LE LN3 (Vin C) @ 273,000 miles
and still going strong (<-Faded sun fried grayish ie needs paint)
2007-Present: ___1993 Bonneville SE L27 @ 210,000
Speeding around So Cal (<-Beautiful Forest Green)
Mods: Sylvania Silver Star headlamps. Remote start system with pageing device. K&N air filter. Tranny Cooler. High-flow cat with Magnaflow muffler. Full Class II hitch setup.