Have a '99 C5 with about 20k miles on it. When I start it up and the fan or air kicks on, for about 15-30 seconds I get a foul odor coming out of the vents. Tried the usual. . .Lysol sprayed into the intake under the hood and also sprayed directly into vents. Also shut down A/C for 3-5 minutes and let fan run before turning off engine. Still same odor. It's kind of musty and moldy and almost "dead animal" smelling at times. But it does go away after 30 seconds or so. Any thoughts?