You say this is a 2004, so that’s closing in on 5 years old. It could be one of or a combination of several things; carburetor to manifold seal, manifold to head gaskets. It could just be the change in air filters. If it is just the change in air filters a jet change may be in order or a readjusting the air fuel mixture screw under the backside of the carburetor.
Are you still running factory exhaust? Did you purchase the bike new or is this a previously owned bike, in which case do you know any history of work done to it? There is a section in this site loaded with carburetor information. Sometimes it is simply a process of elimination, but at 5 years I don’t think it would hurt to change the carburetor and manifold seals/gaskets. This is a relatively inexpensive job and if you have yourself a manual, a couple of Allen heads and some wrenches it isn’t all that complex to do.
I’m kind of leaning on the carburetor seal, and I’ll tell you why, you said you’ve just changed the air filter, I’m assuming here, that you didn’t have this issue prior to the change. In which case I know you had to pull the air cleaner and probably jiggled the carburetor while making the change. If that is the case, you may have broken the seal on a deteriorating rubber seal, thereby creating a leak.