All animals have a distinct scent associated with them. While some are nice and others like skunks are not, rats have mainly just a natural scent. But on occasion rats will have a different scent you may not be ready for.
Male rats will have a nice earthy scent to them if their cage is cleaned at least once a week. But sometimes you may have a rat that smells like warm corn tortillas or corn chips. You may think he got out of his cage and raided the chip bags but this is perfectly normal for him to have a warm tortilla scent.
I had 2 male rats that smelled like warm corn tortillas all the time. The warm tortilla smell seems to happen a lot more often after they have all been curled up together and are warmed by each others bodies or after they have given themselves a good grooming.
Believe it or not female rats do not smell like corn tortillas, I had a female rat that smelled slightly like grape soda. I know it seems wholly unlikely that a rodent could smell like grape soda but mine did. A friend of mine had a female rat and she said her rat also smelled like grape soda. When it came time for a check up, my veterinarian told me that yes indeed rats sometimes will have the scent of corn tortillas, corn chips or grape soda and it's totally normal.
I like both male and female rats and even though males tend to get larger and lazier and a bit more laid back than females, both are great pets. That stereotyping doesn't always ring true either, I had a female rat that was every bit as lazy, laid back and friendly as all the other males I've had so it really just depends on the rat since they all have their own little personalities.
If your rats smell bad, then that is a good indication that you need to clean their cage more often. Rats will pick one corner of their cage to use as a toilet and you can keep the cage smelling better in between cleanings by spot cleaning the 'toilet' everyday. Some rats will urine mark or scent mark their territory, so be sure to watch your rats when they are out of their cage at play time.