A/C vent kinda smells

[Reignhard Stajuana]

Ok guys, I have tried to use the archives but for some reason it keeps
coming up with things that are totally unrelated to my query. Maybe you guys
can point me in the right way to search for it but the problem is that my
ventilation system kinda smells whenever I turn the A/C on. It only does
this with the A/C, not the heater or just the fan. Would using those foam
stuff cleaners that you shoot into the vents cure this? I also heard of them
being used where the vent pulls air from but I don't know where my car pulls
air from. Sorry if this is in the archives and if it is then please give me
the right keywords to look for. TIA

Ray S
98 Sentra SE