If you haven't already found the cause of your "ringing" maybe I can offer some suggestions on how to find it.

Most likely the "ringing" you are hearing is a bearing slipping on its shaft. I fly Concept 30's which will do this periodically. It's usually one of the bearings in the tail drive train. There are a couple of them at the front of the tail-boom, a couple at the back of the tail boom, a couple on the tail-drive shaft. If any of them get "stuck" and start to spin on the shaft you get a high pitched whistle that lasts as long as the heli is turning at speed.

You might find which one it is by putting a drop of oil on the bearings you can see (one at a time) and see if the noise goes away. If it does, it won't stay quiet for long because the oil will spin out right away. You're actually lubricating the shaft so that it spins easier inside the bearing.

It might also be the bearings on the main rotor shaft, altho they usually don't make that kind of noise. It might also be the wire drive running down the tail boom, if someone has installed it in a brass tube (as I have on my Concepts) to hold the wire drive in place. The wire drive problem can be fixed by putting a piece of teflon tubing around the 2mm wire, inside the brass tube. You can also just grease the wire (which works for a while).

In any case, you might want to disassemble the heli far enough to get the bearings out and check them for "smoothness" (and re-grease them while they're out anyway). Check out "The Greaser" available from your LHS or by mailorder for the slickest bearing greasing tool ever made.

If you find any of the bearings where the inner race has worn a groove in the shaft, you will need to replace the shaft AND the bearing. Once it "galls" the shaft its pretty hard to fix it because BOTH the shaft and the bering have been "ruined". You might try to "locktight" the bad bearing to the shaft (being careful not to get any of it into the ball race) for a temporary fix.

CAUTION: This "locktight-the-bearing-trick" can be tricky because you need the bearing and shaft to be "grease-free" for the locktight to work, but if the bearing is not full of grease and you get the slightest dab of locktight into the race, you'll lock the balls in place. To prevent that, it's best to grease the bearing (so it's full of grease) and then carefully clean the inner race surface with alcohol or acetone before you try to lock-tight it onto the shaft. Then you use only the smallest dab of locktight (blue is OK).

As one of my pre-flight ceremonies, I put a drop of TriFlow (light machine oil with teflon) on the two exposed bearings in the tail, on the tail-drive-shaft under the pitch slider, on the main-mast under the washout assembly, and on the main-mast above the main gear so that it seeps down into the one-way bearing. This doesn't help the four bearings inside the tail boom, but it makes the others last longer.

Hope that helps you find your "ringing". If it's making noise, it's WRONG and something is wearing that shouldn't. You will eventually find it while you're digging the grass and mud out of the broken plastic.

Regards,
...paul