I checked the vent line on the house. Everything looks good. I disconnected the drain under the sink. Looks ok. But there is a little bit of positive air pressure coming out of the drain line. Giving us the smell. It started maybe a month ago. Need help?

Is there a trap under the sink that is holding water? There should be and that's what keeps the smell from enter the house.

Some folks don't rinse out their Garbage Disposal with a little bleach. The best way to do this is to fill the sink up half way and put in a 1/4 cup of Chlorine Bleach when the water is almost gone. Pull the stopper and turn on the disposal. This flushing pressure is good for keeping loose food from sticking to the walls of your pipes. Plastic Pipe is really slick so unless you have a belly in the pipe where water stays between each flush you should be OK. Do this at least once a week for good maintenance.

Yes I do Have a Trap.

I do not have a garbage disposable.

It is probably funk built up on the undersides of the basket strainers and in other crevices. If the smell is coming from under the sink, something could be assembled wrong, damage to the vent line, or etc. If you unscrew the tailpieces from the basket strainers, you can use a paper towel with bleach and clean them.