Odd noise in my amp!

[Mostro]

Hi guys, I want to ask you about a noise that recently appeared in my DIY tube amp (or at least, I recently became aware of it). It´s a push pull 6V6 amp, sort of a mutant Deluxe. One pre gain stage, Fender type tone/vol controls, another gain stage, and into a copied-from-the-Deluxe phase inverter, into the power tubes. The biggest differences are, it´s cathode biased, it has a pot for variable negative feedback, the power tube grid resistors are quite large (around 100K, if I remember well), a passive loop, a cap to ground after the last gain stage to tame a little the highs, and some values slightly modded here and there. I can post the schem later if it´s helpful. Oh, and it´s a head, not a combo.
Here´s the noise:

http://soundcloud.com/arifig/il-mostro-noise/s-ghrub

This is a low E, palm muted, guitar>Fuzz>amp. Then a low G, A and finally a low B. All on the 6th string.As you go up in freq, the noise eventually goes away, so it´s frequency related. The amp doesn´t add distortion in the recording, I just used the fuzz so as to be able to record it clearly. It also does it just with guitar>amp, though it´s less obvious. And it takes a little while after the amp is on for it to start. So heat related?
I tried it with different cabs, so it´s not speaker/cab rattle. Is this motorboating? I don´t have experience with that.
Apart from this noise, the amp works very well, healthy amount of volume, all controls work, no hum, buzz or radio stations... Another thing: It´s biased quite hot, around 85%-90% of the rated dissipation of the 6V6´s. That are not actually 6V6´s, but the russian equivalents (6P6S I think).
Well, I wanted to show you people this before I pop it open, to see if it sounds familiar to you. It´s quite strange to me!
Any help appreciated. Thanks!

[Alexo]

Sounds like a mechanical vibration - you can completely isolate the amp from the speaker to test (if noise goes away, then yes, it is mechanical).

If that's the case, I'd first just try a new set of tubes. If that doens't help, I'd do some chopstickin'...

[Mostro]

Update on this guys: After testing various things like swapping tubes, I replaced the 270K (!!) power tubes grid R´s with 2K7´s and the problem went away! Quite possibly I was exceeding the max R allowed in the grid of the 6V6´s, or too close to the limit. So the thing went into blocking, or grid current, or something like that (not too sharp on the inner workings of tubes). Still have to do more testing but it looks as if that was the thing. You might wonder why it had such high value R´s in the first place. It was an attempt to reduce harshness when the amp was first built, by taking advantage of the Miller effect to reduce highs.
Well, live and learn! The moral of the story: Don´t go over the top on those grid R´s.

Thanks to all for the help.

EDIT: when testing, I became aware that the recto tube (5Y3GT) makes a rattling noise when powered (not in standby). Strange, right? It works OK, and has for some time. No red plates or anything strange besides that. Tried a GZ34 and no rattling. Both are Mesa branded. Will it explode soon?

[Reeltarded]

If you are lucky, or maybe it will rattle forever.