Hi, i've had my ball python for about 5 months now, and he is about 3 years old. His name is Salazar and has been an amazing pet. He has never lashed at me or bit anyone, and has always been calm other than the occasional under the breath hiss when he was either molting or when I first go him.
Normally he likes to just look at me, either in or out of the tank. He's funny and will sometimes just come straight out the tank and touch noses with me and stay there for a few minutes before crawling around.
Recently however, when I have gone to clean out his waste he has been hissing at me, and today when cleaned out his tank he brought his head up and stared at me through the tank like usual, and open mouth hissed at me. It was quite distressing and I'm not sure what to do!
Is this behavior normal? Is there something wrong with him?

I use a heating pad under his tank and a timed light to keep day/night time conditions. He is also near my heater which keeps the abient air warmer than the rest of the house, seeing as I live in colorado. He last shed in later october.

Thank you!