My 990 does exactly the same thing. The colder it is the more pronounced the effect. I use the intake pre-heater and the start produces a puff of black smoke and then the engine settles into a high idle without too much smoke and with a fair amount of clatter. After a minute or so, the idle drops and the sound dramatically changes to a quieter, smoother sound with much less clatter. This is also accompanied by an increase in the amount of blue-white smoke. Even if I push the throttle back up to the same rpm the engine started at, the sound and smoke level don't return to the same as when the engine first starts. As the engine warms up, the smoke reduces and the clatter comes back again.
I think what is happening is the intake pre-heater is being kept on for a minute or two and the idle kept up. At some point the electronics decide to switch off the pre-heater and drop the idle. Over the next several minutes the engine heats up naturally and everything returns to normal. I haven't measured the voltage at the pre-heater to confirm this, but it's my best guess as to what's happening.
