[Mesorah] za'aq vs tza'aq

[Sholom Simon]

In Bereishis 18:20 & 21, the Torah seems to use the words za'aq and 
tza'aq with the same meaning (cry, outcry, . . . );  and, indeed, 
Onkelos uses the same word in both places.

What's going on there?

Thanks!

-- Sholom