Rattlecat

See if you can find the dayglow stickers to use as tracking tape. They make a world of difference. I use the pink and green because they are so contrasted, really shows. I roasted 3 batteries, overheated the motor/esc because I didn't set the tracking at all speeds, when I got up to flying speed, it was far out.
CH

Orion,
I think the blades may have been out of balance when you tried the DF36 tail on the MX400. They make a similar sort of buzzing sound. As a mater of fact, they were doing the same on a 36 lately, I put a strip of scotch tape on one tail blade, it got worse, so I removed it, and put one on the other tail blade and my 36 suddenly was so smooth I was amazed. I thought the vibes were from the main rotor. Same thing on the MX a few weeks ago.
Both belts on the 36 and MX400 are the same , a B371T MXL. The MX one is cut a bit more narrow. I think the vibes may come from no undercutting on the Walkera cog pulley. When the bottom of the belt teeth compress around the the cog, they need relief in the cog's tooth profile, meaning a sort of inward cut near the bottom of the pully's teeth. We are paying 1/3 the cost of industrial grade power transmission components. Also, the 36's shaft is probably not true which will show up on the MX due to the 5:1 tail reduction and higher speed.

I went and checked, the MX pulley does not have relief, but they must have compensated by making the teeth more narrow on the pulley so the belt can bunch up. My Walkera pullies are on the helis so I can't check it for now. I did check an Align pulley and it has square teeth, not trapezoidal. And as I remember, they run a square tooth belt. They probably don't want to put up with the teeth chipping on the dovetail cutters. Believe it or not, those filled resins wear the molds out pretty quick, and they are going to have to keep cutting new mold inserts.
Maybe later I'll show a photo of some precision grade pulley teeth so you can see what I mean.

EDIT : You know, as a matter of fact, it seems I remember the Walkera pulley actually having the opposite of a relief cut, a sort of outward curve near the bottom. More like an HTD type timing pulley.

CH