"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.
Well now you've gone way beyond my knowledge of circuitry. Have no idea what an output follower or a buffer is. I can and will try the tap-test as soon as I get some time (pulling out the 16 torx screws from the BAT time after time is a real hassle). I hope that its not a problem with the NOS tubes since I fear that international customer service is not a strong point of ebay auctions.
You should never tap tubes, you can ruin a tube real quick that way. Check out TUBE BASICS AND FREQUENTLY ASKED QUESTIONS on Upscale Audio's website @ www.upscaleaudio.com.
Without knowing why it worked, let me provide a possible solution that was recommended to me and was successful. I had a similar ringing on one channel of my Line-3 when I put in 7308s, and it was recommended that I swap the channels they were being used in. It worked. No more ringing. Try changing the position of the E88 tubes ... seems like an easy fix and a heck of a lot easier than trying to get your money back.
Interesting reading thanks Lp. I'll clarify my definition of "tapping" vs. "banging" as referenced on the Upscale f.a.q. Obviously you don't go in there with a hammer. Go in with some thin gloves like white ones & be gentle & careful. In this case the ringing is so bad that even a slight wiggle in the socket should find the problem. Other places I've read that gently using a pencil eraser-end as your tapper is acceptable. This would produce no more stress than simply moving the equipment around, certainly less than, say, a UPS package handler. What else can you do find this - you just have to do someting? Maybe tap on the circuit board adjacent to the ssocket?
The output tubes are the last devices before the preamp's output; ask BAT or your dealer's tech. for some clarification, as I'm unfamiliar with this products' specifics
Swampwalker: Other than tapping the tubes to check for microphonics, you can also try using 4 Sovteks to two Siemens and trying out different combos of this to find the tube. Siemens E88cc's are not generaly prone to microphonics, but there are always exceptions. Also are you doing the listening with the cover off of the preamp? If so try it with the cover on which will help to block air born vibration. Sounds like you have a tube that enjoys singing along with the music though. Good luck.