Hi all ! New to this forum. I just recently purchased a HH GT150, really is a fun ride! Can't wait to soup her up a bit! I am having a problem with the rear disc making a horrible chatter when applied under a medium load, if you brake hard it doesn't do it. Has anyone else experienced this? The caliper seems to mounted tight and rigid, so if anyone has suggestions would greatly appreciate it.

Looks like your post got missed, If the problem is what I think it is it's a simple fix.

The chatter is most likely caused by the caliper shifting when you apply the break and the pads virbrate. It also happens when pads are worn down unevenly.

If you are 100% sure the caliper is secure then check the bolt on the caliper that adjusts for the parking break. The pads should be very very very lightly touching the rotor. You should also remove the pads and check the metal clips behind them. The crossfires do this all the time when riding for a while.