Have you checked your engine mount bolts? Mine was doing something similar and it was the 2 bolts near the washer bottle loose. A way to test (seriously, but at your own risk) is to sit someone on the front of your car with the bonnet open, handbrake on and put it in first find bite point, then reverse and find bite point to make the engine rock forward/back. you could clearly hear it and actually see the marks on the engine mount where they had been moving for a while. If nothing else, it's a quick one to rule out if you have a willing volunteer :)
I guess in your braking scenario, you could also try diping the clutch before braking (when it would normally make the sound) so that the engine is disengaged and will not rock on the over-run when you hit the brakes, and see if it still does it. Seeing as braking and changing down will be causing an engine over-run, it could well be engine mount related (maybe).
Hey, worth a shot and doesn't get you too dirty. Good luck :)
Stu