Strange noises coming out of my Mark V

[Mike M]

I wanted to see if anyone could help me figure out what is wrong with my Mark V? I am getting a rapid thumping sound in all channels. It only occurs in the 45 and 90 watt modes. It also stops when I select the Tube rectifier in the 45 watt mode. When I switch it back to diodes, it comes back on. In channel 3, it makes the noise in Triode and Pentode selections. The thumping noise is not high pitch, but more like a thud and occurs a couple of times every second. Any ideas what is happening? Thanks very much!

 

[ryjan]

Sounds like interference or a bad power tube.
When the sound is present try unplugging your instrument cable from the amp and anything in the loop. If it goes away then you know it's noise from a cable, pick up, etc.