I too have a downshift "clunk." Mine happens at around 20 mph, like when coasting to a stop light which is red but turns green before you actually stop. If you give it any amount of throttle, the downshift (3-2?) is very pronounced. Somewhere in the archives is my post on this; I was told it was due to a bad B1 piston. I haven't done anything about it, just try to avoid hitting the accelerator too hard at that speed.
I agree with Euro -- I'd get the codes pulled, either by a good independent with a handheld scanner or (gulp!) at the dealer. Paying someone who knows the M119 engine for an hour or two of diagnostic time would be money very well spent.