is my crankcase supposed to be sucking in air while its idling? i mean when my motor is running i pulled the dipstick out and it went "piiiiw" you know like when you put your hand on the vaccum cleaner nozzle at home then pull it off and then it sucks in air. is it supposed to be like that? also i changed my oil cap today, my car was off but when i opened the cap it made the same sound from the oil cap.

please help me cause i dont know if its good or bad.

thanks