Speaker noise after tube amp shut down

Last night when I shut down my tube amp I have noise in one speaker.
This hapen about 20-30 seconds after I power down my amp and last about 10-20 seconds. Today I experiance same.
What you think happend? Bad tube?
Everithing is find when amp is up and playing!
Thank you!
I doubt a tube sounds more like a capacitor discharging. Does this happen every time or only the one time. How old is the amp?
Last night it hapend same, agan noise from speaker after I powered down amp. Maybe litle louder, I'm not shure.
Amp is 5 years old, heavly moded Jolida 502.
Thank you
Exactly what kind of noise? Pop cracking fading ocean wave? Usually tubes make their aches and pains in the rear known when full power to the grid is present not de-energized.
Im gonna say for an experiment switch the speaker leads to the opposite channels and see if the noise follows to the the corresponding speaker. Lto R R to L This will eliminate the speaker for starters.

Noise is like bzzz ing for about 20 sec.
First after powering amp down speaker is quite for about 20-30 sec and after that start like bzzzzz noise for 15-20 sec.
Thank you!
Well, do the speaker lead swap to eliminate the speaker. If the speakers are eliminated then put the leads back to the correct L & R connections and swap the amplifiers input IC's from the preamp now if the sound is still in the same speaker we have determined it to definitely be an amplifier issue and im gonna say a cap is acting up.

For now I would let the symptoms develop and become more pronounced because I may not rule out a tube completely as a failing tube has a way of playing havoc on other components such as caps,
Go through your tubes with a tester for starters but thats usually a long shot

What kind of power output tubes are we talking about?
I did't swap speaker cables yet but I will tonight. Right now I'm not home.
Power tubes are KT88, NOS Genelex Gold Lions.
I will let you know how swaping going.
Thank you!
Got a spare set of tubes? How old are the tubes? When was the last time you replaced them? After swapping the cables, I would try changing all the tubes just to see whether the issue is caused by the tubes or not.

Last nigh I swap speaker cables and interconections from preamp and this eliminate speaker or preamp. Also plug diferent power tubes and problem is same. Only thing wich I didn't is to change preamp and driver tubes. I will swap them tonight and see.
You should swap the tubes between the left and right channel; if the sound moves, then it is in the tubes and not the capacitors. If it doesn't move, I would suspect an electrolytic capacitor. If it moves, then swap the small signal tubes to determine if it is those tubes or the power tubes. Keep doing the swapping until you figure out which specific tube is at fault.

I've had several small signal tube (6sn7) that made noise (popping and whistling noises) on power down, but were otherwise quiet.
Finally I found problem.Problem is bad preamp Brimar 4004 tube.When I pluged different preamp tubes no problem at all.After I power down my amp my speakers are quite like should be.
Thank you all for yours inputs!