E-mailing the PD to express early interest is completely appropriate. I did so and ended up running into them on several other occasions which I think was helpful. If you know someone in the program or there's an alum from your school, you can reach out and ask if they mind talking about the program. An elective rotation to get to know residents an other attendings is even better if you're a good candidate. Having others in your corner when it's time to make a match list is even more helpful and politely and professionally showing interested with a considerate and thought-out approach is good.
Do NOT be obnoxious with e-mails or spam e-mail though. Last fall I received an e-mail from a potential candidate expressing interest in the program I'm associated with. I let the PD know and it turns out the individual not only e-mailed every attending they could find an e-mail for, but every resident in the program as well despite an apparent general interest and having no connections to the program at all. Do not be that person.