highs too high?

i have a speaker box sitting in my back seat with a set of mid range speakers and tweeters in it. the tweeters have a response up to 40kHz. whenever im listening to a song with a lyrics, when the vocalist says any word with an "S" sound it sounds like a high piched hiss  that seems to carry out longer than the "S" sound is supposed to(i guess thats the best way to describe it). could this be because the tweeters have too high a frequency response and i need to put a LPF on them or is it just a bad quality tweeter, or somthing maybe to do with the box?

thanks,

so do you just stare at the forum and wait for new posts all day or what?

dont know what i was thinking about when i guessed at the response being too high... last time i checked i didnt know anyone that could make that sort of noise. is there anything i can do about this or am i stuck? also do you think its the mid range speakers or the highs? i guess i could just disconnect one or the other and see for my self... but making you guess is much more fun.

~Warren