switch the tubes to the opposite sides and see if the popping migrates..then you'll know...yes this is a possible symptom.
Noise such a crackling/popping can indicate a tube going bad but it can also indicate the lack of proper pin contact in the socket and/or a build up of corrosion, dirt, etc. Some times just removing and replacing the tube will solve the problem as you will scrape off enuf of the crud. Other times you might need to clean the tube pins and sockets. Tube noise that sounds like a hiss is meaningless if you can stand the sound. They just are not candidates for positions where low noise is essential, as in phono amps, and some high gain line stages.
Apart from the obvious, the sound of an aging tube is usually dullness, and it can creep up on you. I would suggest that you keep a spare set of tubes around, not only for emergencies, but, depending on use I would periodically change them and se if there was a sonic improvement. If not take them out and try again next year (or whenever).
Oh, another thing, something I experience in one of my amps. There is a very lignt crackling/popping sound for the first 5 minutes and then dissapears 'til the next use. T think that this is due to the tube pin sockets are a bit slack and the heat up process expands the metal and makes a better contact.
Hope that helps a bit.
Purchasing a tube tester can be tricky. There are a lot of testers for sale on Ebay and Audiogon. Many are not calibrated or are in need of some other type of repair. Make sure you ask questions before you buy. Some of these sellers will say it's in perfect cosmetic order, which doesn't necessarily mean that it works well, just that it looks pretty! I actually purchased a Triplet 3414 on ebay that was in very good shape cosmetically and functionally. I was willing to take a chance that it may need to be calibrated, but I lucked out. You also want to make sure that it will test the tubes that you are using or may use in the future because certain testers do not have the facility to test certain tubes. You should also make sure it comes with the user guide and list of tubes, otherwise do not buy it unless you can get a manual somewhere else. You might be able to get an Eico 625, which is a pretty basic unit, for $100-$125 on Ebay. Some of these tube testers are selling for twice the price of what they cost when they were new. It's the law of supply and demand, and I don't think that there is a company that is still making tube testers for the average consumer.
Before you begin your search, check out this web site:
www.alltubetesters.com You will find useful information about tube testers. The guy even offers opinions on which tube testers are worthwhile. He also performs repairs on them, so even if you purchase a tube tester that is functioning perfectly, it may need repair someday. Good Luck!!