I bought a used Peavey Valveking a few weeks back and i have been playing it on the clean channel with a Boss Metalcore pedal (cant afford a tubescream yet). I got new 6l6 tubes in the mail today and just retubed it a few hours ago. It has been sounding great and working just fine and then all of a sudden it made a random loud popping noise when i was playing it on a very low volume. It sounds just fine still although i haven't played it much since then for fear of doing any more damage. does anyone know what could have caused this and if its a serious problem?
If it wasn't doing this before, then it sounds like you may have an issue with an arcing tube. Or, it's also possible you're overdriving the speaker and it's bottoming out - in other words, the voice coil is reaching the end of its travel.

If it's an arcing tube, you can reinstall the original tubes and determine if it stops making the popping noise. If the popping noise remains AND it's occuring while you're playing at loud volumes, it's the speaker's voice coil. This is your warning sign to turn it down and not drive it so hard. If you continue to overdrive the speaker, it will damage it. Either turn it down, or replace the speaker with something better.

If you determine it's an arcing tube, you need to contact the vendor you bought them from and request a replacement. Typically, an arcing tube will create a static-type noise, which leads me to think you're overdriving the speaker.
My amp makes a pop when I change channels and play at the same time. Would this be much of the same?
No. That's actually something different. Some amps will pop a little when you change channels. It can also be caused by a faulty preamp tube. My Mesa Lonestar used to do that after I bought it. However, it was an extremely loud pop. After a couple of weeks, I turned it on one day and nothing - no sound at all. Found one of the preamp tubes had gone bad. I think it was the phase inverter. Got a new one and the popping cleared up.

When your amp pops during a channel change, how high is the gain/volume?