Can the police search my car without my permission, if they smell marijuana coming out of it?

In Mass, new caselaw has held that it depends on the odor of the marijuana--and whether there is a "fresh" smell or a burnt smell. 

The general rule is that if the police are going to search your “persons, houses, papers and effects”, they will need a warrant. However, there are general exceptions to this requirement, including an exception that applies to searches of cars. Of all the exceptions to the warrant requirement, this exception is the most board.

The US Supreme court has decided that motor vehicles deserve a reduced “expectation of privacy”. The general automobile exception allows a police officer to search a car when the officer believes that there is “probable cause” that the vehicle contains evidence of a crime (i.e. the investigating officer has a reasonable belief that a person has committed a crime and/or that evidence of a crime or contraband would be found in a search). This means that an investigating officer gets to make the on scene decision as to whether he has probable cause to search a vehicle or not.  So if the police say they smell marijuana coming from your vehicle, you’re in a tough situation. Courts have ruled that the odor of contraband gives officers probable cause to perform a search.

Note:  There is nothing you can do about an officers decision to search at that time. However, a good rule is to object to an officer searching your vehicle. It is not going to stop the officer from doing it but it will provide you with a better case if, and when, you get to court. If a judge decides that there was no probable cause to search the car (and that no other exception applied), then the evidence will be suppressed and the case may well be dismissed for lack of evidence.