"Ringing from pre-amp

I just swapped out the Sovtek 6922s from my BAT VK3i pre with 4 NOS Siemens E88CCs. I did NOT replace the 2 Sovtek 6V6s. After about 4-5 hours of use, the pre has developed a significant "ringing" sound, a steady pitch tone that cuts out when I mute the output. It is not a hum, but a clear tone like you would hear if you struck a crystal wine glass. It is not present immediately after the mute-on-start delay kicks out, but appears within a few minutes thereafter. It is loud enough to be heard distinctly in the background with moderate level music and is quite noticable without any volume. I also notice a significant level of what I will call microphonics (when you tap the top of the case without any input (but not muted), it produces a sound thru the speakers. It occurs most prominently when you tap the case over the 6v6s. The unit is also running warmer than before. The sound, however, it more detailed and little more forward, with excellent bass. I did not get a chance to contact BAT today. I would also note that also I recently installed a BAT factory phono card. Any help would be most helpful. I am not an experienced electronics tech or only moderately handy. I do not have a nearby BAT dealer.
David- the "ringing" starts off on its own and is almost a feedback like tone although not nearly so high in volume or pitch, nor does in "feed back" on itself in terms of self-perpetuating volume increase. The "microphonics" that I am talking about is a thump thru the speakers when I tap with my fingers on the top of the case, which is heavy steel. It varies in volume with the intensity of the tap. It seems to me that I should see if I can repeat the problem with the Sovteks back in the circuit. If its still there, then I guess I need new 6V6s ASAP. Any suggestions on the best for a pre-amp application? Most discussions I have seen address their use in a guitar amp or some such, rather than a pre-amp.
Swampwalker, before you discard any "culprits", try installing a set of tube damper rings. It might be all that your singing tube needs, and will help all the tubes sound their best. Good luck.
Thanks for the tip, Froggy, I guess all of us swamp creatures need to stick together. FWIW, I swapped channels on the input tubes and the ringing has stopped, even tho the microphonics are still there. I have thought of tube dampers but then I read that some say that they "deaden" the sound somewhat. Any thoughts out there?
If removing unwanted "ringing" and false brightness from the sound of your system constitutes deadening the sound, then yes, they will do that. They will also allow more detail to be heard since there won't be as much high frequency garbage getting in the way.

Good luck.