We moved into our house 2 month ago. For the last 3 weeks, whenever the stove fan is on, there's this odor coming out one of the bathrooms. We just thought the odor was from the cooking. But tonight, while the stove fan was on, I realized there's air being pumped out of the washer drain pipe in the bathroom and it made the whole bathroom smelled like waste. We turned the fan off, and the smell subsided in 20 minutes or so...

The house:
Brand new with 2 bathroom, one in the basement and the other on the main floor, both has w/d hookup. We put the w/d in the basement. There's no smell in there. It has been raining and we have a huge tree out front that has been dropping leaves like crazy. Maybe the leaves clogged up the vent pipe? Should I call a plumber? If it's not the vent pipe, what should I expect. Please share your thoughts. It'd be much appreciated!

What you are describing sounds like somebody connected stove vent into plumbing vent. You have to investigate to make sure it is not connected that way. Stove vent should be dedicated vent and terminate on the outside, usually on the side of the house. Plumbing vent goes straight up and terminates above the roof.

Could be what Milo says but it could also be that the fan causes negative air pressure in the home and the house sucks air inside through whatever means it finds. A really air tight home with an exhausr fan has to pull in make-up air somehow. Your washer connection may not have a trap or may not be sealed. Tell us what you find.

Agree...

Also, it may be toilet with bad wax ring and fan pulling in sewer odor through toilet base...

Is there water in the traps new house?