Sounds like your dog is feeling nervous or uncomfortable or stressed or afraid or anxious about something if he is growling and has stiff body posture.
I'm not quite sure that punishing/correcting him by putting him in the basement is such a great idea. You say he acts "guilty" but I would guess that he can read your body language and he knows you are unpleased by his growling, and so he is sending you body language signs to appease you (or make you not hurt him in his mind)
He is probably fearful of something or someone's past reactions and so he is growling to let you know how he is feeling.
It is best to really think hard about what happens at the moment he is growling. And what happened right BEFORE the growl. Figure out what is stressing him and work on changing his association/mindset about the issue. Be glad your dog is communicating to you by growling!!!
That is a very good dog in my opinion. You are right to worry about an impending bite in the future. Dogs that get punished for growling, or have owners that ignore the growl, can tend to learn that it is not worth it or safe to growl-- and so they can go straight to biting to get their message across
. Your dog is doing good now to be still trying to communicate with you... now it is your job to listen to him and acknowledge that something is worrying him. And then work on reducing his stress so he will not need to be worried and hence growl.
When my dogs growl, I always acknowledge their growl and uneasy feelings and try to reassure them that we are ok, or I give them space, or I take them away from the thing that they are growling at. Or all three. Then I get to work on a plan to counter condition them to all fearful/stressful situations.
For example, last night, my shy cautious heeler mix Gracie was growling a bit at my newer high energy pup Puma last night when pup was bounding across bed in front of Gracie.
This makes Gracie nervous at times. I get it. She gets uncomfortable with other animals being too close to her, especially with very fast movement.
So I told Gracie very gently and nicely "Hey Gracie, we're ok, I got this, no need to growl at Puma."
Then I redirected her over to me, away from Puma, and gave her a short massage. She calmed down and stopped growling, but since Puma was still bounding about, I then called Gracie to kitchen and hooked her up with some treats while playful Puma was in bedroom with Dad with baby gate. Problem solved. Gracie relaxed-- and hence had no more need to growl or escalate. Puma pup played rambunctiously with Dad, and Gracie and I hung out in kitchen
Knowing why your dog is growling, and then acknowledging that your dog's stressful thoughts are legit
are the first steps in helping your dog overcome his anxiety.