Hi,
I think the really important part of this "problem" is that it may not be a problem at all. You said:
" When I put ear close to the speakers I can hear what sounds like something frying in a pan"
If you had to get that close to the speaker to hear it, and if the noise does not increase when you turn up the volume, then don't worry about it. Just about every amp, tube or transistor, can send a little bit of noise into the speaker. Tubes always send a little noise, that increases with age, which is one of the things that can tell you when to replace them.
Don't worry about it.
I think the really important part of this "problem" is that it may not be a problem at all. You said:
" When I put ear close to the speakers I can hear what sounds like something frying in a pan"
If you had to get that close to the speaker to hear it, and if the noise does not increase when you turn up the volume, then don't worry about it. Just about every amp, tube or transistor, can send a little bit of noise into the speaker. Tubes always send a little noise, that increases with age, which is one of the things that can tell you when to replace them.
Don't worry about it.