wah-wah sound

Not sure if this is a dulcimer problem or a playing problem.

Now that I have my new dulcimer strung, I've been learning "Ode to Joy" on the melody string. But it seems that especially when playing notes on the 1st, 2nd and sometimes the 3rd fret, I hear a kind of "wah-wah" sound. Is that because I'm not pushing down with a constant pressure or is it because there is a problem with one of the frets?

Any other problems that beginning players seem to have when starting out?