"Ringing" vibration at certain speeds
 

I have a 2001 XJ that I bought back in Nov. Ever since I've bought it, there's been a ringing noise from what I believe might be the braking system.

The best way I can describe it is the sound you get when you wet your finger and rub it around the rim of an empty wine glass. It only happens at certain speeds, typically 25, 45 and 65 or so mph.

I'm curious if anyone else has had a similar issue with their XJ and if they ever found a solution. I've heard all types of issues between something to do with braking system and/or even the bearings in the pinions. But what gets me is that if it were with a bearing (wheel or pinion) or something to do with stuck piston or shoe wouldn't I hear it 24/7 and not just at certain speeds?

As an fyi, I've already replaced both front wheel hub assemblies and bearings about a month ago. I've also pulled both pinion yokes and replaced, each I could not find any visible wear etc although I know there's no way for me to tell if inside bearing is toast without pulling out the entire assembly.

Any help is appreciated.