The RJX was going to be certificated as a variant (it was also going to have a common type rating with the RJ). The certification was never finished although most of the trials, certainly on the RJX100, were completed.
Many attempts were made to cure the "flap-hoot" but most were at best partially successful. I believe it was thought to be caused by reverse flow around the underside of the exposed part of the wing when the flaps were travelling through 12deg on their to 18deg.
When I started working with the 146 I was told the aircraft was so quiet, you could hear the flaps being retracted after take-off. Wow! I thought, not realizing that it was the flaps that were noisier than the engines for the wrong reasons!
JT