I would agree, not a break in characteristic with V30 speaker. If you said it had Jensen Blackbird Alnico speakers and it breaks up oddly or you get some really weird noises, that would be considered normal and it takes a while to break those in.
Buzz could be a number of things and may not be the speaker. It could be the baffle board, grill, or even the casters or something else not even cab related. I would consider relocating your gear in a different location to see if it is something else first. I have chased the buzz around with an OS Recto Cab to no avail. Found baffle to be causing issues and fixed that. Sometimes it would go away and then come back the next day. Tried the same cab with another amp and did not have the issue. Trial and error, I found the footswitch enclosure tends to vibrate on some frequencies as I was able to stop it when searching for the root cause of buzz when I put my hand on the footswitch I had sitting on the floor next to the cabinet. Since the Vertical cab directs one speaker upward, it is easy to assume it would be the top speaker. Try moving your amp into a different location. May even want to move your head away from the cab if at all possible as it could also be tube rattle that may sound like it is coming form the cabinet. Sometimes you may need to remove the grill in order to chase down cab buzz. It could be the plastic stopper support for the grill causing the vibration. Soft rubber foam door seal tape can fix the buzz if it is related to the grill frame against the baffle. Removal of the grill will at least isolate that from the root cause. May not be easy task to track down. May also need to glue in some tooth pick stubs into the old screw hole if the wood on the grill has become stripped out by the screw. I get buzzes every now and then with all of my amps, I have to consider what room I am in when it happens. My wife had decorated the home with many glass and ceramic objects that tend to resonate and rattle which has made me think it is related to the speaker cabinet. Definitely try what you can to isolate the noise if you have not done so already. Does it make more buzz on a clean channel vs higher gain setting? If so, it could be fret buzz. If you have more than one guitar and it still happens, may even be tube rattle as that can sometimes get into your signal. I hope some of this helps. It could be the one speaker too but I would do what you can to rule all other issues out first before replacing speakers. If you have to replace the speaker, buy it though the mesa store as they carry the England made versions. The others are made in china but look the same and do not sound quite the same as those tend to buzz or get fizzy at low volume (was a common complaint in the Carvin forums about the V30 before Carvin audio split from Keisel/Carvin guitars).