Apple Logic Pro /LogicExpress Discussion

I'm trying to mix vocals into a song and I can't seem to get rid of  
some essing sounds, and what I can only describe as a sort of 'silver'  
sound that comes and goes at various points. Anyone have any tips for  
finding the source of the problem? Sorry to be so vague about how it  
arises but I can't hear it on the clean unprocessed vocal line

thanks in advance

Andy B