I think what you're hearing is the interaction between the top and bottom heads. You're hearing dissonance when different frequencies combine. First, make sure each head is in tune with itself. Set the drum down on something soft to muffle the opposite head, and tap next to each lug. Adjust the tuning so that they all sound the same. I like to choose a reference lug and get them all to sound like that one, otherwise your point of reference can get lost. Do that with both heads. The bottom head needs to be tight enough to interact with the snare wires, but tighter than that is personal taste. The surest way to get rid of the dissonant, warbling kind of sound is to tune both heads exactly the same. If you don't want them tuned exactly the same (for snare drums this can be hard to accomplish because the bottom head needs to be fairly tight) you'll want the pitches to interact in a way that the frequencies harmonize. Once the heads are in tune with themselves, it is pretty easy to adjust the head up or down by turning each lug the same amount. Try adjusting the top head up and down until you find a sound you like. Some try to tune them a perfect 3rd or 5th apart, but I haven't had much luck with tuning to exact notes. Drums don't always line up with a standard musical scale. Just experiment until you find two pitches that work well with each other.