My MK3 is about a year old and I have only recently began to tube roll.  About a week ago one of the stock 6922 began to loudly click as I raised and lowered the volume.  I know it is the tube because I moved it to the other side and it began to do the same thing.

Moving ahead.  I was able to use another set of 6922 the other day and one side began making a hissing-type sound.  Almost like someone breathing into the microphone (difficult to convey).  It was more-or-less intermittent, nevertheless annoying.  Again, I isolated the problem with one tube only.  

Do 6922 (input tubes in general) last?
Is this a common problem?
Have any of you experienced issues like this and is this just the norm with tube amps?
What 6922 or input tubes seem to be long lasting?

Any advice is appreciated.