Strange noise coming from my air vents?
 

I have a '97 Blazer and lately I've been noticing a strange noise coming from the air vents. Now I know that the AC has a leak (not sure where), and the noise only started happening after I recharged it and it ran out. It is like a farting noise and usually happens when I turn the fan off. Any insight as to what this could be? Thanks.

i think thats just the vacuum discharging but not really sure without hearing said fart