That all depends on whether you think y is a vowel or not, as it serves a vowel on occasions, and zyzygy - what does that mean??? - is one of those occasions. If, for the sake of argument, we assume that y isn't a vowel, then RHYTHM is another 6 letterer - or as a plural, 7, as mash101 points out - and then you have SKY, MY and THY. Leaving out the y then you are probably limited to sounds, such as SHH and HMM. However, you could make it a long SHHHHHHHH, or very pensive HMMMMMMMMMMM, if you really wanted to go for a record.