Fokker 100 cold and dark APU start 

No need. Fully automatic. If I recall correctly the temp indication was either optional or related to which APU model was installed

DowntownCanon , its been a while since when i flew the F100, but as a general rule, since the aircraft is powered on batteries and its the batteries that will start the APU, before the start procedure you have to check that the batterie has enough charge to avoid a dead battery in the middle of the start cycle. If I recall well the minimum voltage that guarantee that is somewhere around 24 volts

I think ITS due eletrical Gyros spining and coolers If you are saying about noise after bat is on.

Inverters

@@brazil_3931 The big noise after it started is probably the trim warning