This problem is pretty common if you dont know how to set up a snare drum.
I had this problem with an old kit I was doing up to give to a friend, buzzing and the snare sounding loose, it sounded more like a tom. This was because the snare wire (the coils on the bottom) wasnt tight enough against the bottom head, this may require you to drop off the snares with the mechanism, and pull the strings or plastic/ fabric or whatever strange TIGHT, theyre attatched at the other end (held under a small metal plate) - pull it tight here and hold it tight until you've screwed it back into place, then push up the strainer mechanism lever and twist the nob thingymebob to further tighten the snares.
ALSO make sure the snare strand is the right way up in the first place, if it isnt, it wont touch the head as well.
If you dont know what youre doing and dont want to mess up, get the drummer to do it, if he doesnt know, he should learn!