Loud Creaking Tongue & Groove Roof

We recently built a new addition onto our eichler house from the 50's.  The old part of the house has 1 x 10' tongue and groove (with a fake groove down the middle).  So when we did our new addition we wanted to match it as much as possible.  It is a new garage with a master bedroom above it.  The new 1 x 6 T&G is nailed to 4 x 10's that are 32" on center.  The roof is about 550 sq feet.  4 to 6 months after the roof was completed it started to crack and pop quite loudly and pretty often.  It typically only does it during the night than during the day (but it does happen during the day).  It is definetely loud enough to wake you up.  I am sure it has to do with the contraction and exapnsion of the wood with the heating and cooling throughout the day.
The way the roof is constructed is doug fir t&g nailed to the beams.  Then there is osb sheets over that with 3.5" rigid insulation panels over that.  Then there is another layer of plywood (per the engineer) over the insulation.  Then there is a layer of dens decking for fireproofing and the last layer is 1.5" of spray foam that is painted tan in color.  The roof is not vented either is there is no dead air space.
I guess my question is does anybody know a solution to fix the problem?  Could we have constructed it differently?  Added vents somehow?  Spaced the T&G further apart. 
Thanks
James