I agree, it's probably a bad tube.....next would be contact points like purple tele said. Also check your speaker connectors.....Check every electrical junction: switches, pots, jacks, etc and clean them with Deoxit...
"People sleep easy in their beds because rough men stand ready to do violence on their behalf."
“To forgive is to set a prisoner free and discover that the prisoner was you.”
Location: "Deep in the Heart of Texas" (Near the Alamo)
V4 definitely. That is the tube 4th from the right. It is the tube that recovers the sound from the reverb circuit and mixes it back in with the dry signal. The others don't sound bad.
Using a screwdriver to tap on the tubes is kind of risky. You stand a good chance of cracking the glass. Using something with some padding on it you can strike it harder without that risk. Like I noted previously, I much prefer a pencil with a slip-on rubber eraser. If I don't have one handy I use my fingertip. I posted a video awhile back of the RCA factory where they made tubes back in the '50's/'60's. They had a test fixture where they whacked the tubes pretty hard with something similar to that pencil to test for microphonics. I was struck by how hard they hit them.
Anyway, that tube need to be changed. If you don't have a spare tube handy, for test purposes you can swap V4 with V5. V5 is only for the oscillator for the tremolo so it won't be affected by the microphonics. I'm not sure what Z put in V4 in your amp but a 12AX7 will work for testing until you can get the preferred tube.
Oxidation is a nasty and elusive little bugger. As a routine, I hit every point of contact with DeOxit when an amp with some age comes to my bench. Once found a Jaz with some very very subtle hum. (I'm a hater of hum and it just doesn't belong.) I use a nylon adjustment tool (same one I set bias with) to apply pressure to key areas to search for issues like this. The culprit in the Jaz? The speaker jack mounting to chassis for ground. I removed the jack...DeOxit to the jack...small piece of 440 grit cloth to the inside chassis where the jack mounts...reassembled...no hum.
Some people use popsicle sticks or chopsticks...be real careful using those...if you get some high humidity and they soak a bit of moisture...they can conduct. Use a nylon probe...won't have an issue.
If i'm not pulling the chassis and just want a fast assessment...I'll soak a 1/4 plug and insert & rotate...problem gone or lessens? You know what the culprit is. But, don't try loosening & retightening jacks without removing the chassis...twisted wiring is not your friend either.
i've had a half dozen or more Z's in for work...what I love?? Everything...good construction to get to anything you need...lightweight...no 30-40 lb head like many steel chassis Marshalls (I do a lot of vintage Marshalls in this SW Ohio area...Marshall Major has a weight!!). I was impressed enough with Doc's work that I swapped for a Galaxie a week back...love it. Yes, I'll mod a little, but love it.