1995 Grand Marquis AC smell

[tjb99777]

I have a 95 Grand Marquis that has a smell that has come from the ac vents two or three times in the past 3 weeks. It did not smell like anti freeze and it occurred immediately after I started the car after having driven it with the a/c running and then letting it sit for 3 or 4 hours in the sun. Upon restarting the car--this smell appeared and then after about a minute it was gone with the a/c running at full blast. It was a rotten sort of smell. It has just come and gone. Any ideas as to what it might be? Thanks alot! TJB99777