Figured it was a bit rude to hijack the drill press thread so this is a new one.
Is the "ringing" sound possibly a "rattle" from the clutch plates??? If it goes away when you engage the clutch and comes back when you disengage the clutch it is likely that it's simply clutch plate rattle - a common occurance as I am told. Mine does it more than my buddy's - I have heard that some guys will lay a bead of silicone across the top of the clutch plates a "anti rattle" feature.
There is a rattle with the clutch disengaged, so I should probably look at that, though I rarely use the clutch.
The ringing I referred to is really more of a low frequency rattle that is more pronounced when the leadscrew is in forward or reverse and seems to mostly disappear when it is in neutral. It sounds a bit like a large washer rotating loosely on a shaft. It's hard to tell for sure but it sounds like it may be coming from area of the plate, hub and handwheel, to the right of the upper belt pulley. Or possibly from inside the gear box and getting transmitted to the those parts through the spindle.
The only "problem" with mine is that the VS knob (crank) will slowly "unwide" when it is turned off for any length of time. Obviously I have a bad seal or checkvalve in the mix, I've just been too busy to track it down. I figured it's in the upper piston assy most likely as I recall I replaced everything in the lower piston when I did the pulley. It holds position whiel in use so I guess the annoyance factor is so low it may take a while till I get to it
I'm not sure about the upper seals, but the lower ones were only a couple of bucks each from Clausing and probably much cheaper from a local supplier.
AS for the vari speed only going to 50% of range - One thing I noticed when reassmebling mine was that you have to "orient" the upper and lower pistions to one another (forgive my poor choice of words) - In another words, the pistions and pulleys have to be calibrated to one another - AS I recall this is accomplished by adjusting the nut on the shaft that protrudes out of the lower VS pulley set. Tightening it (I think) Increases speed - loosening it reduces speed (or vice versa) - I believe it is mentioned (skimmed over) in the manual.
I tore mine apart to move the lathe and vaguely recall having a problem with the upper cylinder parts "springing" apart. The fluid was drained, refilled, flushed, and refilled again and bled repeatedly. I'd suspected that I'd just gotten the upper piston position incorrectly, but will check "calibration" as you suggest. I'm pretty sure I went through the nut adjustment when the pulley was replaced but it's time to look at the whole thing again. My work tends to be small so I'm usually at 1,000 rpm or higher and fixing the range has been kind of low on the priority list.
If none of that worked then I would suspect a belt that is the wrong length.
The belt is in good shape and still bears the Clausing name imprinted on it. The lathe is 40 years old and shows obvious signs of use so I suspect it was a fairly recent replacement.