You often get quite a lot of inudction noise. How is your air intake arranged? Generally, sound will eminate from openings like a big spherical ball - is there a direct line of site between the intake/sound meter? If so can you shroud the air intake on the one side to prevent the sound travelling in that direction. You should bear in mind that at the edges of the shroud, the sound will 'wrap around' the sharp corners but at a much reduce amplitude. Another little thing is that sound levels are not straight additions - best to look here
http://www.engineeringtoolbox.com/ad...ibel-d_63.html
but basically if you have two noise sources, that are the same sound pressure level in dB, you will get a 6dB increase in the measured level.
Hope that makes some sense