One or two hisses or growls are fine- if you are hearing more in one play session I would watch them closely.
If they trade off who is attacking and who is defending then all is well- if it is always one going after the other, then watch them more closely.
If they are friendly after playing all is well. If you are consistently seeing one cat stalk the other, or one is continually hiding then watch them more closely.
In all probability all is well. How old are they? Are both neutered? Do you keep their claws trimmed?
I try and stay out of "animal business" like this, but I do observe them pretty closely to stay aware of what is normal for them so I will notice if things go awry. My three are 10 months old now and they play pretty roughly, but I rarely hear any squealing. If I did I would pull out a lure toy and redirect their attention rather than physically breaking them up, unless as Satai mentioned, there is blood or real fighting.
Real cat fights are pretty noisy, and it is a sound you won't mistake- you'll just know that it is not right.