The car is an 88. The belts are OK and I spray them once in a while. I did not check the smog pump, but normally the pump works along with the motor so I should have the noise intermitently. In my case it happens during mornings too, I didn't pay much attention to the outside temp when it happens. Once the engine is restarted later on during the day nothing happens. The noise is very present in the alternator area, IMHO the air filter has really nothing to do with it, the housing maybe. The sound cuts off sharply as if some sort of vacuum system was cut off. I couldn't find any vacuum hoses diagram in the shop manual.
1973 porsche 914 2.0