Interesting idea from my friendly local mechanic today.
Suspected air leak on the air filter, inlet side of a car.
With the car running spray the INLET side of the engine with burst of WD40 or similar.
When the leak is sprayed the engine note changes.
Easy!
NB Before anyone else says it. This is a stupid idea. Your vehicle, home, family and geraniums will perish if you try this really dangerous procedure
Actually this reminds me of a technique for finding porosity in a carburettor body, as shown to me in a BMW (car) dealership’s workshop manual … 'fact I think he gave me photocopies of the pages, but which I haven’t kept …