I'm sorry but this sounds more like the drive belt is eating the rear sprocket! If it is that high pitched and it sounds like a drill is trying to go through your ear drum, that is where I would look first. The sound will carry through the seat belt access which is why the ears take a beating. It, too, will come and go depending on the load.
Your accumulated kilometers is a big tell as well. Check for black powder covering everything back there as well as slip a phone underneath and photograph the sprocket where the belt is off of it. If the teeth are noticeably eaten and deformed then that is your culprit. The black powder is oxidized aluminum off of the sprocket. Morgan is now making them out of steel which works very nice. I have the Super Max overlay which also works.
I have a 2012 bevel box and it whines and I notice it as it is attached to the frame right where your hip sits. The addition of Red Line oil makes it behave. I have been changing it out at every oil change.
I hope this helps. I went through three sprockets and the noise on all three change outs was amazing! Bevel box can't even reach those levels. If it does then look back along the road for bits and pieces of it as it self destructs!
