I went ahead and located a good, supposedly NOS 6JS6C tube for my Zenith Color. It's an RCA Brand.

The tube looks brand new, and it acts brand new except it makes audible noise that go away with in the first couple minutes of use. Screeching, and some squealing.

Is it gassy? Or is it shorting out? It only starts to act weird when the high voltage kicks in, and then it passes and stays silent as it should. Even when it fusses and makes noise, it's working correctly and the TV has no negative reaction to it.

Did I get a dud?

I went ahead and located a good, supposedly NOS 6JS6C tube for my Zenith Color. It's an RCA Brand.

The tube looks brand new, and it acts brand new except it makes audible noise that go away with in the first couple minutes of use. Screeching, and some squealing.

Is it gassy? Or is it shorting out? It only starts to act weird when the high voltage kicks in, and then it passes and stays silent as it should. Even when it fusses and makes noise, it's working correctly and the TV has no negative reaction to it.

Did I get a dud?

Some tubes I've noticed will get a little debris on the filament which will cause the tube to make funny noises until the debris burns off (I had a similar situation with a NOS RCA 6BM8 tube which when I went to put it into my Motorola Record player the record player made a loud humming noise which when I took the tube out and shook it a little bit and popped the tube back in and turned it on, the tube flashed a little bit and then started glowing like normal and then the record player played normally again).
Try taking the tube out and shaking it a little bit and then popping it back in and turning it on again and seeing what happens with the tube.