[rotorwrench]

When you are going down hill, the car transfers from engine driving to engine braking. A number of things happen during engine braking. The rear axle starts to pull on the torque tube instead of push as in normal drive. All of the engaged gears start to ride against the opposite sides of the gear teeth both in the rear axle and the transmission. The U-joint and even the clutch disk will have a different torque load than it does during normal driving. The engine uses compression to brake with no accelerator applied so this changes the load on the exhaust valves when they start to open so there is a lot of stuff happening in a different way than it does under normal drive.

Vibration also tends to have some noise associated with it but it may be difficult to tell where that may be coming from. Rear axle & drive shaft can get some vibration during deceleration. The transmission is generally OK unless there is frontal alignment deviation or a bad bearing somewhere. The current problematic new parts in the transmission are the countershaft rollers and the tip roller between mainshaft and input shaft. Some are crap. Old original Hyatt rollers with normal wear are preferable to the new stuff. I can't say what parts are used in the Mitchel transmission but it doesn't hurt to know that all the early Ford transmissions from the model A up through the 1950 Ford F1 use the same caged roller bearings. Mercury cars from 1949 through early 1951 used them as well.

If the vibration is up front then I don't know what to say about that. The Burtz engine is not something I have any experience with. If it's sheet metal type vibration then I'd look at the engine pans or something of that nature. Sheet metal vibrations can get loud and are usually sympathetic type vibrations that are caused by other sources.