1999 Volkswagen Beetle

One day this summer i scraped underneath my car and it started unusually vibrating. When i turn the air on it immediately starts to vibrate and when the air is not on i can feel it when im at a red light i can feel it through my brakes like a fun is running or something...So could be the engine? transmission, radiator fans, or possibly i need a front alignment checked. Also, i have had this checked this summer and the man said he couldnt hear it but it still is there? Wat is it?