Wonderful news! I've got it working, replaced TDA power amp, and miraculously the noise disappeared! Clean is crisp, everything's working...
I also thought that external would cut the speaker, but that IC got fried, and it seems to be the good thing.
I suppose that it was faulty part, oscillating in a wrong way, cause that noise didn't come from some mechanical reason, like loose part that's rubbing of something inside, it was more of "electric" nature...
Thank you all again for taking interest in this.
Cheers!

Thank you for your advice.
Will try to follow as soon as I get it fixed, because in meanwhile I managed to burn it.
I was trying to see if the problem is the same when I plug in the speaker as external, but didn't detach the speaker before. Well, when the smoke cleared, I saw that this TDA2030 part got burnt. This shows how much I (don't) know about circuitry...
Hope that it's the only part that got damaged, speaker is fine.
Thanx again...

Hi, and thanks for reply.
I've attached another amp to speaker thus eliminating speaker itself as an issue.
The noise is more of a artificial nature, such as the dirt that's being made by some kind of IC, but this resonating is mysterious...
And it's interesting that it's less of an issue when the phone is attached. Could it be that there is a ground connection missing somewhere so that the phone is doing some kind of grounding, or it's battery is doing something?

Hello,

Few days ago I bought this amp as a second hand, barely used. Reviews said that it has a good clean tone. Anyway, I didn't expect much, it works well, nice sound but... There's some kind of noise, like resonating dirt, depending on a pitch of a note, and it only happens on lower register. It's not reverb related, it's not the speaker. With headphones there's no issue, and it seems that there's less of this noise when I plug in my phone to CD input..
Maybe the problem sounds familiar to you guys, perhaps with other SS amps, I have no idea what could it be (except that it's just built that way, part failure, poor part quality or loose connection...)
Would be gratefull for any feedback on this.
Thank you,
U