My 85k Mile 855T has been creating an odd new sound at cold start-up with increasing frequency.

The best I can describe it as a muted hoover vacuum sound. It is not engine speed dependant, it does not change with A/C compresser cycling or an increase/decrease of the alternator load. It seems to be coming from the serpentine belt area of the engine. It usually stops after 15-30 seconds, but in more prevalent in colder temperatures.

Any ideas?

Sounds like your air pump (part of the secondary air system). If it's getting louder than it used to be, it may be starting to fail? The air pump is located under the battery.

The air pump forces air into the engine to bring it up to operating temperature more quickly. It always will come on if you don't warm up the engine before driving. I always hear it come on whenever I start driving without a warmup on cold mornings. The first mile of my daily drive is easy and slow, so I let the engine warm up on that stretch of road (you should never push a cold engine past 2,500 RPM's by the way...just not good in the long run)