Before I make a mountain out of a mole hill and run off to the vet I want to make sure this is an actual problem.

My enchi isn't the friendliest snake in the world and just loves to hiss at me. Well 3 times within the past week I've seen bubbles coming out his mouth after he starts working him self up hissing at me. The first time I wasn't sure what I was seeing and they were gone before I could get a good look at them. The second time they came out with his tong and today he had a nice big bubble on the side of his mouth.

I've checked his mouth several times and I have noticed anything other than a little saliva. Against my better judgement I've even put him near my ear to listen for any popping sounds and I haven't heard any. He's been eating fine and just had a perfect shed on the 10th. He looks and acts normal.

Are the bubbles just from him hissing so much or is there something potentially serious going on here?

Other relavent commonly asked for info:
He's in a 41qt rubbermaid tub in a rack
paper towels for substrate
two hides which he uses
eats once a week
temps are 90-91 on the warmside and 78-80 on the cool side
I don't have a humidity reading, I'm currently working on that, I'm guessing it's around 40-50% there is no condensation on the tub and his water dish is on the cool side