I would change the stock tubes out. Keep them just in case for backups but I'd bet it would be time for you to get a 2nd set of new tubes before you'd have to use the stock tubes again. It could also just be the tube retainers rattling a little bit. Try some e-tape to keep them from vibrating.
Sometimes it's to reduce/prevent noise pickup, sometimes it reduces microphonics and/or instability (as with the Vietnamese Hand Wired AC15s we have - I added shields to V2 and maybe V3 - I forget - as they were a bit unstable when cranked).