Odour bomb

got a funny smell coming from the vents now and then, has anyone tried or can recommend any odour bombs, also where would you place the bomb in the Clio?
 
It's when I turn on the air con, for the first 10 to 15 seconds I get a funky smell, but then goes, if I turn the air con air off and just have blower going the smell comes back again. Weird
 
Has the air con been serviced recently? Could be a potential cause of the smell if it hasn't been done?
 
Dont waste time or money with chemicals, take it to a place that can do a proper clean with ozone, this will not only remove the odour but clean the whole cabin and make it germ free, you have no idea how much nasty stuff is in our cars!