Can someone tell me what wrong


Hi there
i need some held with speaker hissing noise
i recently noticed some noise come from my right speaker 
the speaker is venture audio grand mk3 and driven by vta m125 tube mono block, it’s kind of loud, i can hear it from 4 feet away the speaker with the preamp turn off, it doesn’t effect by the preamp volume 
so here is what i did
Remove the preamp - still the same noise
swap the mono block - still the same noise
swap the speaker cable between left and right - still the same noise
hook up a vintage dynaco st70 - no more noise
hook up the right channel mono block to the left speaker - no noise 
so what could be wrong here?
I didn’t have that noise before 
This thing really drive me nut
Thank in advance 
 

128x128ownut
I had a similar problem with my tube amp.  After giving manufacturer a hard time about it turned out my outlet wasn't grounded and way amp was designed the ground is important.  Wasn't a problem with previous amp because it was vintage and two prong plug.  Ran a ground wire directly to fuse block and solved problem.  Can get one of those $5 outlet testers to check and see what manufacturer says about it.
After swap the power tube the problem still there so i swap the 2 driver tube and still the same
i then looking at the power circuit, find out that its the same circuit which i recently move the wifi router 
Unplugged it then the noise is gone
A very stupid mistake 
thank for helping me out with this

Way to go noromance! I’ve got an Ethernet switch in my system. Though I’ve not noticed any problems, how do I know? It’s always been there. Will a switch potentially cause cause the same problem as described above? 
@mbw Any electronic device can mess up the sound. In this case, it was a wi-fi transmitter which is as bad as it gets! Listen with your switch on and off. Any difference? That's your answer.