I once found an annoying crack in a manifold using an Insense stick
Light the end and use the smoke is the first indicator but when you get close to the leak the lit end glows brighter, proper
but it worked! I guess a firework lighting stick will do the same.
If you've still got the grotty viscous fan you'll have stop it from turning or all you'll end up with the the feint whiff of "tree huggers living room"