Cat Behavior Question

Question
I currently own two cats (but there are four living in this house all together). My male cat, Sassy, is about 2 years old and my female cat, Kat, is around 9 months old. Both cats are fixed. Most of the time they are the best of friends!. They snuggle together and give each other baths.
Some times they become VERY aggressive towards each other. Sassy usually initiates it and they end up biting, hissing and growling. Sassy stands over her and makes a strange clicking noise with his mouth.
It seems to be happening more often since we spayed Kat.
Are they playing? should I be concerned about their behavior?. They are so cute when they snuggle!.
Thank you for your time, I appreciate it!.
Pictures:
Happy times:
http://img.photobucket.com/albums/v308/amber581/SASSY/b835f5a8.jpg
Bad times:
http://img.photobucket.com/albums/v308/amber581/SASSY/1f7a559f.jpg

Answer
Hi Amber,

Loved the pictures! Their behavior is nothing you should be concerned about. It is totally normal. It's a "Hey! You're in my space now back off!" behavior. For whatever reason one of them will think the other has over stepped their "territory" boundaries and is smacking the other down. Sassy, being older, feels the younger one is crowding him too much. It will happen, just let them settle it out. And as they get older they will do it less and less. Good luck.

Ciao, Karen